Marathon training involving not running may sound odd at first, but it is absolutely necessary.
In fact, to a large extend my twenty week marathon training has the non-running as a integral part of the training plan. Many reasons why this is so, but the key one is to allow muscle recovery.
In addition for muscle recovery and growth, the off days are for exercises to increase strength and flexibility of the joints, tendons, and other key pieces. I also use the opportunity to research the refined topics of running such as nutrition, speed development, etc.
Today is an off day for me, and I plan to do some hip and ankle exercises.
What do you do on your non-running training days?