Diet, Exercise, Genetics. You can control two of those.
The longest distance I run with any regularity is two miles and most of my work are 800 meter repeats and some sprint work. Have you tried one of the concept rowers? What about swimming?
I warm up with body weight exercises. Push-up, pull-ups, dips, chins, sit-up, v-ups, and some mobility drills. I keep my lifts simple. Front Squats, Bench, Military , Lunges, Deadlifts, Rows, and Power Cleans. I will do a variation of those exercises in some form or fashion. I suspect your back may prevent you from doing some of those. You can lift heavy if this is too easy and add weight to your pull-ups and dips.
I do a boat load of mobility work (mobilitywod.com). It keeps me in the game after years of beating my body up.
That is an over simplified version of what I do, but it gives you an idea.