Well, I went through Pathfinder, Ranger and Airborne school in l954. I don’t believe the training has changed that much, except for rappelling out of choppers etc.
OK, you need a certain amount of upper body strength. Push-ups, pull-ups, sit-ups – you should be able to max out on these.
A high score on the leadership reaction course also helps.
I ran in HS and college so that was the easy part. You should be able to run 8-12 miles cross country carrying a weaon and possibly a pack.
You can build up to all of this now but you should be pretty much at peak performance when you enter this training. Its not easy and much of it takes place when you are tired, wet, cold and hungry. Um, after I finished jump school, I never jumped out of an airplane again. And since I ran for the army in Europe, I never really used the Pathfinder or Ranger training-but it was fun and not that tough if you were a skinny runner. It was the football types that couldn’t complete the training runs etc. Good luck.