I mentioned this above. If your races are consistently 3+ hours and you are consistently blowing up around 2 hours, and you are fueling well, the only thing left here is that you are going out too hard and trying to maintain a higher effort during the first 2 hours that leaves you smoked during the rest of the race.
It sounds reasonable, and that’s what I think is happening to me, deep down. I suppose there’s no training that can help me handle that initial effort better than slowing down a little.
I suppose it’s also about finding the right balance between the energy you expend in the first part and the last. Don’t start out too strong so you’ll still have energy left at the end. Sometimes this is difficult when you have to position yourself to avoid bottlenecks on climbs up rough trails…
How about trying to pace significantly differently for your next race? You could set yourself power ceilings for the first long climb, and — depending on the course — set targets for the others segments.
You could test the thesis that in the past you pushed too hard in the beginning and see if an alternate strategy works better.
I agree with a lot of what’s being said here.
Some of your recent races show an IF of .89 for the first 90+ minutes, and .80 for the first 3 hours and 40 minutes.
It’s tough because the style of modern off-road racing typically starts hard and whoever fades the least wins, so pacing can be tricky.
You do have some data points to reference with your past races, though, so I’d maybe use them to develop a plan that you could stick to for your next race. Based on what I’m seeing, maybe trying to stay under .85 for the first 90 minutes and under .80 for the first three hours could be a good place to start.
Another option would be to look at your power curve from the past 6 weeks for another bit of data to reference. 
Fitness changes from day to day, week to week, and month to month, though. You’ll have good days and bad days, so take everything with a grain of salt.
First of all, thank you to everyone who has taken the time to help me.
I think we can conclude—unless @eddie disagrees—that my training itself is well structured (that was my primary concern), and that nutrition is unlikely to be the main issue. Instead, it seems that what I really need to work on is pacing and managing my effort more effectively during races.
That actually makes sense to me, and it’s something I’ll focus on in my upcoming events.
I would highly suggest trying just straight sugar with some form of salt. Push up to 140g to see if your stomach will tolerate it well. It’s made a world of a difference for me.
If I tried over .8 IF for 3 hours and then tried to keep riding I’d probably drop dead
I just came home after a 3h 20 min training session with an IF of 0.875 and TSS of about 260, albeit from a possibility conservative AIFTP but never the less I’m pretty cooked!
If I’m to add something to the discussion that is to increase the training volume. The OP seem to have everything dialed so that is pretty much what is left. Durability is the latest buzz word and that is basically a high level of fitness or CTL in a power context.
+1 on this if you have the desire and time to add more volume. Pushing .8 IF for 3+ hours should be very reasonable without blowing up if you are trained for it. It sounds like nutrition is solid. I agree with some of the other posts about changing your pacing approach, but at some point it’s just a matter of other racers being fitter and/or more talented even when you are doing everything right. That can usually be fixed with more training (can take multiple seasons/years), but most amateurs have limited time/energy to train.
Question:
What’s your FTP and what was your NP during each hr of your last race?
I’ve had the same issue for the past few years. Always lose power around 4 hrs into the race regardless of varied pacing strategies I’ve tried. Nutrition is dialed, recovery is good. I’ve stuck to trainer road plan for 7 years now. Big improvements initially but stalled out in improvements the last couple years. Part of it is I’m 48 and understand I’m near my ceiling. That said, regardless of all the talks and podcasts and reasoning behind low volume training working for longer races, it not only doesn’t make sense but it doesn’t work for me. Only until this year when I switched it up and took advice from a local coach and increased volume with specific z2, tempo and threshold intervals in my longer weekend rides that I started seeing big gains and able to hold power for 6+ hours. You may be able to get enough adaptations with low volume plans to complete long races but I’ve found it’s nowhere near sufficient enough to match what you need to be successful. I’m sure loads of people will disagree but this has been my experience.
This makes perfect sense really. Even if the physiological adaptation piece could be argued (I think its silly to argue with it but even TR claims you don’t need long rides, I dunno, sounds silly to me, but anyway…)…even aside from physical adaptations the mental practice and pacing experience is likely as helpful as anything else anyway.
I say its a no brainer: if you want to get good at a thing… DO THAT THING!
You wanna get good at riding long? Do more long riding!!!
Interview with head of UAE performance
Tadej normally loses less than 1% of his fresh performance despite having done 4,500 kilojoules of work, which is why he’s able to do these long attacks late in a race and get away and stay away. Other riders are starting to have that fatigue and that’s one of his key attributes, his ability to have exceptional durability at a very high workload. In Tadej’s case, his durability is exceptional.
…
The other part is also training. So my former colleague [at UAE Team Emirates-XRG] Iñigo San Milan introduced the zone 2 training, and that zone 2 training is specifically targeted at enhancing the ability of the body to use carbohydrates and fats, but also very much the carbohydrate side in an oxidative way. In other words, to use them within mitochondria [inside cells] to produce energy, which is manyfold more efficient than using them glycolytically, which is when you produce lactate.
So enhancing that capacity is a key part of our training regime as well, the zone 2, what we now call metabolic training. We’ve shifted a little bit. I’m going to stop talking there now because otherwise we’d tell our competitors too much, but we’ve specifically focused on that as well.
But there are so many different facets to performance. It’s really a lot of different layers that we add into the cake.
Sure. I’m around 292w FTP. 47 years old 68kg
1st hour 263NP IF0.90
2nd hour 240NP IF0.82
3rd hour and few minutes till finish 220NP IF0.75
You’re not the only one that recommend me that approach! The point for me including (let’s say) two a week Z2 endurance long rides is what happens with other planned days of TR interval training? What will happen if I accumulate fatigue and failed the over unders or long umbral trainings? I’m pretty sure TR adapt and cut the intervals length or difficult… As you see, I’m pretty lost here: trust the IA or trust the experience in the field. If the second, why I need TR if it doesn’t adapt to my needs and doesn’t understand my weakesnes? Maybe is not for me? I’m making divagations.
With no doubt, this make sense. But maybe is not that simple. I will love to do only long rides. I’m pretty sure I will enjoy it more than struggling with VO2MAX, over unders or long umbral rides. But we need interval training to improve, isn’t it?
So, TR handle this concepts well? Can we trust the process and stick to the plan with the security that we are doing the best training for our goals?
The physiology of pro cyclists is very different and superior to that of even gifted amateur athletes. For instance, there was a study that compared how pro cyclists deal with heat compared with regular athletes. Pros faded much less, if at all, whereas non-pros faded quite a bit. (I remember a few years ago where someone argued that Jan Ulrich “lacked durability” when he battled it out with Lance Armstrong in the TdF.)
Pogacar is a stand-out pro athlete, and I would be hesitant to derive many lessons for ordinary athletes like us. Adding zone 2 training or shifting emphasis towards Z2 means something very different for a pro whose sole job it is to cycle and has a team around him to help him succeed.
More training only makes you faster if you can recover.
No one with a sain mind would say that… its about balance. Do the intensity and the long rides, you have to balance to two.
But could we just include intensity into some of the long rides? I feel like a few long rides that also include some intervals are a lot more engaging than going easy for hours at a time.
Agreed.
Can’t remember where but I saw a youtube/podcast something a while ago with a pro coach who talked about the riders doing 4hr rides with sweet spot or threshold efforts at the end. Basically a 3hr endurance+ ride and straight into cloudripper 
But then I’ve also heard coaches say that doing sprints at the end of endurance rides mutes the endurance adaptation.
Its likely just a matter of degrees and your goals and both things can be true.
The physiology of pro cyclists is very different and superior to that of even gifted amateur athletes.
I would be hesitant to derive many lessons for ordinary athletes like us
Just hear me out. Pros had to start somewhere as they weren’t born with durability and the ability to put down power after 3000kjs into their rides. Maybe there are things we can learn from their training and implement it into our own instead of immediately throwing it out because we arent pros… yet 
Personally, I would prefer not to have a limit put on my training due to currently being an amatuer. Who says we can’t train like that, given that we are able to recover, like you mentioned (which is key)?
Totally agree with this except that pros filter to the top largely aided by absolutely top shelf genetics that enable a crazy amount of stress and incredibly fast recovery. I suspect that’s what Oreo meant by physiology. There is more to getting to the those kinds of numbers than just effort, sometimes you just need the right parents and the genetic disposition to punish the hell out of your body and bounce back quickly and in most cases no amount of pure dedication (absent the genetic assist) will get a rider there.
But maybe you have those genetics! In which case absolutely get after it! 