A few things to do while on real roads that may supplement trainer time:
The latter is as much a mental workout as physical. Getting on it time after time and not letting it drop off too much between, so maybe 70%/95% ratio, roughly. Then being able to keep doing that after hour 3.
Thank you @DXR for the valuable feedback.
Indeed, pushing power downhill goes long way. I’m able to do that for the first 2h, but during hours 3 and 4 that task becomes increasingly difficult.
I also tried to stay aero for as long as possible to the point that after 4h my neck, shoulders and back hurt. perhaps I also have to look at the setup of my bike. But aero works. I gain usually up to 4kmh by going aero on the flats. More so, while descending.
I haven’t tried yet the “FTP effort” during a ride. I think it’s worth considering.
While I ride, I try not to let myself go below 30kmh on the flats. When climbing, that’s different.
This year I reach more often speeds aver 40kmh on flats, compared with last year when seeing 35kmh was out of ordinary. That’s encouraging.
I haven’t yet shaved my legs, LOL. That’s next.
Well theres your problem 
I’m in Chicago…the only hills we have are expressway overpasses. Coasting down hills isnt exactly an option lol.
Indeed @Abe_Froman . At 1st I thought that I could somehow translate a 2h sweetspot workout in TR to a 3h ride outdoor, power wise, But I was wrong. Outdoor, there are many other elements impacting my ride, the wind being the most important, then the rolling hills that never end. The surface of the road is also important.
Here is another thing to consider. I touched on it in a post on the thread about rocker plates.
Riding on a trainer is different to riding outside - your body doesn’t need to work to keep the bike balanced; your brain doesn’t have to think about corners, road surface etc.
Whilst these things aren’t ever going to be your primary use of energy, then on the first few outdoor rides after a long period on the trainer, they are significant imo. After a few decent rides outdoor, it does dial in quickly, but I (& many others I know) have all commented on how hard the first couple of long outdoor ride is after a long winter on just the trainer.
I totally agree @ivegotabike . Besides a high power output to overcome a headwind for instance, riding outdoor requires a certain adaptability to the elements. I have only 4 rides outdoors this year. I am on a HVol plan, but I’ll replace the weekend workouts with outdoor rides. That will probably improve my fitness faster than through the TR workouts.
Perhaps try doing some time trials outdoors? Pick a nice quiet stretch of road and start riding out-and-back TTs there frequently. Not only will you get quality workouts done but you will also get an idea how much your position and pacing will affect your overall speed and power output.
Power doesn’t really matter, the fastest one wins - always 
Give it a couple weeks, “riding” inside is not the same as riding your bike outside. And when comparing training/competing from 8 months ago to today, was it on the same courses? Same temps? Start with that before questioning anything else.
funny how riding your bike more builds this type of fatigue resistance and capacity to do more work… and to think TR high volume isn’t actually high volume.
@Helvellyn and @shiner79 have your answers.
I think you are downplaying this.
Aim for a more consistent training plan next autumn/winter and you’ll continue to improve imo.
That’s a good idea, @Vpe . Indeed a 10-20km TT should help with pacing and position.
Indeed @WindWarrior , once I went outside and had these long rides, the TR HVol didn’t feel like a high-volume anymore 
The rides this year are different than the ones from last year. Last year I rode about 10 rides of 100km, mostly in town, with not many windy days . On the open road, in the country side, things seem to be different.
I think you’re right, @JoeX. I’ve got valuable advises through this post, which I’ll try to incorporate into my training plan.
Not being consistent enough, that was an observation that surprised me, but that would explain my struggles. To incorporate short, 10km…20km TT rides, that’s also a good idea that could pay out.
3–>3.4 W/kg is a 13% increase. I would do bad things for a 13% jump on my W/kg in 8 months. And the FTP jump is 25%…again, I would do bad things.
As others have mentioned, the day over day repeatability is a major boon.
Moreover, riding a bike is a SKILL. You get faster the more you’d do it irrespective of your power. As I have learned first hand, any old idiot (=me) can smash themselves on a trainer and fly up a hill…it takes some chops to ride smooth and fast (=the 70 year old who has been riding for 60 years blowing by me on descents).
Please take this as an objective feedback and not a criticism - I can’t help to notice you also gained 8kg during that period - this has some small effect on your surface area and definitely is noticeable uphill. Even with higher W/kg you simply need to put a little more force with accelerations on climbs etc. which is probably contributing to amassing the fatigue just a little bit more than a lighter rider would at the same W/kg.
Having said that, getting faster or even staying just as fast in a hilly terrain at 10% more weight is pretty impressive!
Thank you @konradkowara . Indeed losing some weight it would definitely help. I’m following a nutrition plan.
However in my case there is a fine line between being hungry all the time, losing weight but also losing power and the other side of it : not being hungry , having enough power, but not losing weight.
Totally agree, @The_Conductor . An outdoor ride is affected by many little things than can’t be learned riding on a trainer. That’s why, pros don’t ride indoors
For me this is a lesson learned.
LOL, those 60-70 years old lanky guys are indeed pretty fast
This is me at 60kg, 300ish ftp (bit higher probably), not at all flat course. Canyon Endurace bike, 51mm wheels, no skin suit, no aero socks. 264 normalized power. I train 6 days indoors and do one of these outdoor efforts per week.
Training inside has many benefits and many advantages over outside. I do 85% of my cycling indoors due to time constraints and do one big ride every weekend. I also do some workouts outside, time and course permitting. For myself, I see the benefit of translating these structured workouts to the real world where even riding up long climb doesn’t result in a smooth and steady pace. I feel like if you are not getting calibrated with some shifting winds, deviations in elevation and terrain and weather, then you won’t be ready for your A event.
As for endurance rides, I recall doing what I planned to be a Z2 ride outside that lasted me about 3-ish hours. I worked hard to stay under sweet spot and tempo and also to keep power in my Z2 range as much as possible. That meant easy uphill, pedaling downhill, squeezing my brakes when coming to a stop, etc…by the time I got home and off the bike, my legs were legitimately tired. Even though I ate throughout the ride. That was about 6 weeks ago. I’ve been doing these endurance rides pretty much every Sunday since and last week I came home last Sunda after almost 5 hours, and I still felt pretty fresh. Keep in mind, I don’t care about my average speed or distance, I mostly look at my NP, calorie expenditure, active time pedaling, and total time and I gauge how I feel the ride was.
That was a long way of saying that you need to bring your structure outside and give yourself some time to allow your power and fitness to translate to the outdoors. I don’t think you can substitute that unless you have a long history of riding indoors and outdoors.
