Hill training strengthens the legs for running, better than any other activity I know. At the same time it can help you maximize your stride length, increase leg speed, and improve  your ability to run hills in races. The hill training period provides a gentle introduction to faster running, while improving your capacity to perform the speed work later in the program.
In my 52 week plan* hills are gradually introduced into the program, while the long runs are increasing. Complete rest (by walking down the hill) so that injury risk is reduced to a very low level. A day off from running is also recommended after running hills.
*Learn more about hill training in Jeff’s book Running – A Year Round Plan  at: https://shop.phidippides.com/collections/books-dvds