You ain't all that much older than me dude. Maybe about nine years. 
Yeah, that's about when you start feeling it (at least I did). The last year I was stationed in 29 Palms, a freind of mine and I climbed up Mt. San Jacinto just outside of Palm Springs. It took us about 3 hours from a base camp at 6,000 feet to climb to 10,900 feet, a total of about 7 miles. I tried doing that same trip back in 2008 with my GF, and my knees gave out about 2/3rds the way up. I had gotten out of shape of course, but I didn't realize how much.
I'd need to get into a full time workout program for about 6 months before I could think about playing Infantryman again.
As far as getting a new rifle goes, I like the AR-15 upper chambered in
.264 LBC.
