Well I've been lifting weights off and on for the past few years. But recently been playing a lot of football. I want to change my workouts from basic lifting to something towards football training. I usually play running back so i tend to do a lot of sprinting. I know i can't be like the nfl running back but i would like to incorparate the same excerise they do. Should i increase the speed of my workouts to build endurance? Any help would be greatly appericated.