The most important day in any running program is rest. Rest days give your muscles time to recover so you can run again. Your muscles build in strength as you rest.
Hal HigdonThere's one rule of thumb that suggests that you need one day of recovery for every mile run in a race. Another rule of thumb...suggests one day...for every kilometer run in anger.
Hal HigdonFinish: Even if you run a slower than expected time, you succeed in any marathon when you finish.
Hal Higdon