I have kept purposely vague how much time to spend biking, swimming, walking or whatever, wanting for you to maintain a low stress level. Don't be a slave to any training schedule, this one or any other. Having said that, if you take too many days off you will compromise your training program.
Balance is essential in any program for success.
Four miles at a comfortable pace. Yes, that's one mile further for this day than I have asked you to run during the first six weeks of my marathon training program. It's all part of a gradual buildup to more mileage. Other than more mileage, another reason I schedule changes in distance is for variety, so you won't be running the same distances day after day after week after week. B. It's a mind thing.