I biked the longest I have in my life; my previous record was one 12 mile ride, but today I went ~36 miles on my MTB. Should there be any steps I take to recover myself? Also what things should I inspect on the MTB? I noticed that during my ride there was sometimes pain in my lower back ... should I be moving the seat forward or backward?
You cannot draw any conclusions from one ride. Especially when your have ridden triple the distance of your previous longest ride. Anyone would expect some discomfort in that situation.
Regarding recovery, it should have started during the ride. By this I mean you should have made sure you ate periodically and drank frequently. Some things that can help recovery
- a massage by a sport masseur
- catching up on fluids
- a good meal
- a good sleep
- a recovery ride
- stretches before your muscles cool completely
- and (I previously omitted) a cool-down period before you stop exercising.
- also, sports compression garments (especially shorts in this case) will help your muscles recover.
Should there be any steps I take to recover myself?
Depends on what is your goal.
Do you want to ride hard the next day, too? Then absolutely, there is a full spectrum of options to speed up the recovery, as in the comment of andy256. The most important are probably a good meal and a good sleep. Avoid alcohol, too.
Don't care so much about riding hard the next day? Thirty something miles is little more than a warm-up to more experienced cyclists... just live your life normally.
Also what things should I inspect on the MTB?
If you have no reason to suspect a specific problem, there is no need to inspect anything in particular. Like usual, before next ride, check the tire pressure maybe.
I noticed that during my ride there was sometimes pain in my lower back ... should I be moving the seat forward or backward?
Typically lower back pain indicates a too-stretched out position on the bike, so you may try to move your seat forward. It's also possible you have a bike that's too large for you. But like Daniel R. Hicks said, a single observation is not enough to draw definite conclusions.