It only comes with experience and "SEAT TIME". After buying my SG last spring ( my 4th Harley) I would go out every weekend and go farther than the last. I started out at around 500 then went 650 the next weekend I went 750. Now this is on a Brawler solo seat, which is less than 2" thick and a 2" tall windshield!
In September I did a Iron Butt 1000 in 15 hours. If you stop often you will get sore, and each time you stop it will get worse until your used to the miles. On the Iron Butt there was 9 gas stops and 1 nice officer thought I needed to stop for about 20 minutes, of those Ionly got off the biketwice. Gas and Go!
If your gonna go for the Iron Butt, get a tank bag so you can keep some food and water or gatoraid close without having to stop. A couple sandwiches, some beef jerky, a couple power bars go along way.
Make sure you adjust your bars and levers to be as comfortable as possible also. They ma be OK now but try some different positions to see if another one is better.
Ride safe and dont push it, make sure you've got enough tunes to keep you from getting bored also!