As far as fit for combat goes, stamina and endurance are most definitely the key. The NUMBER ONE indicator of survivability in combat is your level of physical fitness. Being more fit lets you perform better and, at least as importantly, gives you the strength and reserves to recover if you do get hit.
A couple years ago, when it became apparent I was going to be taking a trip to the desert I finally got back into regular workouts. I don't have and equipment so I just went with pushups, situps and running. (What can I say, I'm unimaginiative)
After several months I got up to around 75 pushups, 90 situps and and 2 miles in ~14 minutes. I'm mid 30's. I actually put on weight doing it, but slimmed down a bit. Had I NOT done this I very much doubt I would be here. I did wind up spending quite some time in the hospital, losing about 50 pounds (among other things.
I can't emphasize enough, if you expect to be heading to combat, get in shape!. Don't worry about how much you can lift, reps, etc, just work. Most especially, run or do some other cardio for endurance.