Picking up on the marathon analogy, once you are in shape to run a marathon, you don't need to run one every single to remain in that shape. Shure you need to run to keep in shape, but not a marathon.

Also, when you train, one day you might lift and some light cardio, the next heavy cardio and light lifting, the next a long bike ride. Your guitar practice should be like that too - add variety and don't be going full tilt every time. Concentrate on speed one day with a bit of theory. The next day, maybe have a long theory session and just do a few speed runs to keep things limber...and so on.

Variety will also keep you from burning out.