It very much depends - how long have you been running for, and what kind of mileage have you been doing? If you're quite new to running, you might be safest keeping your training at around the same level as it is now until the schedule catches up with you. If you've been going a bit longer, then you might feel comfortable staying ahead of the schedule and getting a couple more 20 milers in than would otherwise happen. Stepping back to rigidly follow the schedule doesn't seem like a great idea - if you're comfortable doing 10 - 12 miles, you really should keep that going.
Either way, you'd be wise to follow the general shape of the schedule, e.g. cut back weeks slightly when the schedule does.
