Cardio... which I do none of and Abs (Core) will support your back... which I also do none of
oh yeah, squats for you knees... which I do none of
I guess what I'm saying is that you will gradually build the muscles needed to do the job... just the other day on a live show, the utility was asking me if I wanted my dock... I said no. It was more work for her to drop the cables and fly the dock over for me to rest for all of a minute and the run it back off the set and get back to me before the count down. Pop open the vest and put the sled on the shoulder is what I do and she asked me was I trying to be He-man? Nope, just didn't see the need for it... I wasn't like I was exerting energy standing at rest, now IF I had just jogged up a graded incline on a two minute take... NO TIME TO GET THE DOCK!!! TAKE THE RIG!!!
But IF I was to hit the gym... defiantly the core muscles and get use to stretching like a runner... THAT will help tremendously.