Agree - this is the exact type of thing a bike fitter is there for. I do plenty of ridiculously long rides and a good bike fit is essential. It’s common that you can get away with something for 3 hours, but in hour 6, or hour 10, everything goes to hell! A bike fitter can also help you figure out the type of saddle that works for you.
Don’t double up on your chammy - that will just cause extra sweating and chafing.
I just wanted to add that if you have localized sit bone pain, you might be doing something strange or have your saddle tilted down a lot. On a high performance saddle, we don’t sit on the sit bones despite some shops and saddle makers suggesting that we do with various measurements. Here is a pelvis sitting on a saddle.
Note how the sit bones aren’t taking any weight. Most saddles have a dip in the middle and you sit there and slide back in the pocket and push against the rear of the saddle. We sit on the bone at the base of the pelvis.
More info here:
Personally I love Assos shorts because they have heavy padding in this center section where we actually sit.
That does make sense. Probably why it is not common practice then
So many votes for Asos. I measured up my current chamois against the pressure points whilst on the saddle and INDEED they are about 5mm too narrow in the meaty part. Time for a change
That is VERY encouraging!
Looking into this: so many different ASSOS bibs. I think the Mille GT C2 may be what I am after. Which one do you wear?
Those look good. I have a similar set from the year before. I have three other different sets and they are all good.
One other thing that might be factoring in is any pre-existing muscle tension that’s affecting your posture on the bike. I can comfortably go ~6 hrs without much in the way of discomfort, but then on occasion on the same set-up will get the kind of thing you’re describing with rides of ~2 hrs. Generally this happens when I haven’t been giving enough attention to stretching and rolling in the days beforehand.
Aside from any toughening it will also improve a bit as you get stronger. More power (at similar ish cadence) going through the pedals results in more of your weight being supported by your feet vs your butt.
I’m envious of you; my MTB butt pain starts around 40km in. I’ve had a professional bike fit, changed my saddle to the one he recommended, and yet…40km is about my threshold. I do use Chamois cream, which I think helps, and I also try to stand up frequently to periodically relieve pressure, and yet…40km is my threshold.
I recently started gravel riding, and I thought things would be better with the different handlebar positions, and yet…40km is my butt pain threshold in gravel too.
I still do 40km+ rides, because dammit, but I know that from 40km onwards I’m going to be uncomfortable. I’d love to be able to make it 75km before the pain kicked in!
It’s all in the name 
I’ve used the same pair of cheapest dhb bib shorts for ten years. I also have a variety of tri shorts bib longs etc. I’ve settled on Huub, a bit influenced by the Brownlees.
None made any difference whatsoever to pain. It was and is all entirely down to sitting properly on the saddle imo. I thought I’d cracked it in the first year. Then I shifted a bit and thought I’d cracked it in the second year. But it wasn’t until the third year I got it right. Including two “professional” bike fits.
Then I got off the bike after 8 hrs in 2019 and found blood on the saddle and running down my thighs…! Back to the drawing board.
I’m in my tenth year and I still sometimes shift around in the tri saddle and wonder if I’m doing it properly.
So it’s a game of degrees ultimately is what I think I’m saying.