Look no further than acw4red's advice. Girl knows her stuff. When I blew out my knee, I recall my rehab being heavy doses of stair stepper and stationery bike (is it stationary or stationery - one of 'em is fancy paper. Clearly that's not the one I mean). Anyway, I digress...

The bike is a great workout for your knee. I also remember it being of CRITICAL importance that I strengthen the muscles surrounding my knee: Calf, Quad & Hamstring. You really should include some gym work in your recovery. Good ol' fashion leg curls, leg extensions and calf raises...Can't beat 'em. As acw4red said ~ perfect form is paramount. If you're not sure, don't be too proud: Hire a trainer or look up a place like Rock Valley Physical Therapy. A knee is nothing to screw around with.

After your strength is back, squats, step ups, etc etc. I blew my knee out in 1995 - completely severed PCL, partially torn ACL, torn MCL and torn meniscus. I never had it operated on. I followed the direction above and for almost 20 years now I've been pain free without incident.

Good luck and thanks (to you and to Mielke) for your service to our country! Welcome back to MTB'ing!