If you are planning to run a marathon, you will be ready at the end of that period to begin my 18-week marathon training program. If you are training with no immediate goal in mind, simply continue what you've been doing. Today is a rest today.
Variety is the spice of life.
I've asked you to run this distance. There should be no question about your ability to run this far now in a workout. The only question is, will you be able to run today's workout with a smile on your face, because you're well trained. Non-runners sometimes claim that they never see runners smiling and use that as an excuse not to do it. It's a lame alibi, but make them liars.