I'm hoping someone can offer some advice.

I love long distance cycling, and plan to do a 200 km brevet within the next year (with plans to do 300 km, 400 km and beyond as my schedule allows).

I've done 180 km+ and regularly ride 80 - 100 km+ at a time (multiple times a month).

My ass, hands, and legs are totally fine until around 130 km, then saddle pain creeps in.

I will admit that doing these distances on a folding bike with 20" tires isn't ideal, and perhaps that's just the limitation I have to work with. I do have a suspension seat post and wear padded shorts on those long rides, but again, everything is quite comfortable until around 130km.

Is this a bike fit issue? A padded shorts issue? A small tire issue?