the sled can be used for a variety of things, gpp, conditioning, rehab, prehab, hypertrophy, and strength. when i ruptured 3 vertebral discs about 3 years ago, while on the mend, i used heavy sled dragging to maintain strength and size as i could not squat or deadlift for some time.
i am toying withthe idea of using the sled and a prowler for my primary low body training, and only doing squats and pulls(loading my spine) once every 10-12 days.
here is how i have used the sled in the past:
gpp/conditioning 8-14 trips of 200 feet, short rest periods between trips(1 minute or less) with 1-3 plates. this consited of low body and upper body pulling pushing and dragging frontwards and backwards.
rehab: same as above, but this would be in lieu of what i would otherwise be doing in the gym.
strength/size: instead of a squat or pull workout, instead i would use the sled as an alternative heavy low body training. several trips of 20-25 steps, or upper body pulls or pushes, starting with 1 plate and going up each trip by one plate until failure. rest between trips 1-3 minutes.