The only thing you have to do is be in somewhat reasonable shape, be able to go on 4(-) hours of sleep a night, and be able to go off of one “meal” a day – unless you’re a winter ranger, then you get more sleep and 2 MREs/day. I was a winter ranger and it’s honestly the best time to go IMO. Some guys were buying MREs for $25-$100 a pop. I never sold mine, but I never needed them all either.
The biggest thing is the mental aspect and by mental, I mean being able to put up with the BS.
Walking up mount Yonah(?) with a 240B (might have been an M60, but I don’t think so – it was several years ago), Tripod, and some extra ammo was about the closest thing to a kick in the nuts I can remember and the closest thing to what I expected from Ranger School day in and day out. We did a couple of “movements to daylight” that were tiring as hell, but other than that, I was honestly disappointed. Mount Yonah was no joke and broke me off on the movement in. After about five times of denying help with the 240B, I finally sought some about 3/4ths of the way up and going from a 240 to an M4 was like going to a weighted bat to a whiffle ball bat.
Making it past the first few days is key too. It’s where the RIs do some pretty shady crap to weed out the “weaker” elements because they themselves cannot keep up.