I destroyed both my knees mountain biking (or actually doing a slightly controlled 30 foot drop on my bike, still got bloodstains on my truck seat from the drive home

), and was actually scheduled for surgery to repair a torn meniscus in my right knee, but a couple weeks before surgery they did one last MRI and it turned up clean, somehow my body had fixed itself. Yes, I am a miracle.
But anyway, definitely watch your knees, and your back, those are definitely sensitive items if you want a career in the military, especially if you're going to be in the field, they're going to get enough of a beating after you join, no need to start early.