You should be able to comfortably run distances between 3 and 6 miles. The key to the program is the long runs on weekends, which build from 6 miles in Week 18 to 20 miles in climactic Week 4. You can skip an occasional workout, or juggle the schedule depending on other commitments, but do not cheat on the long runs.
Rest is an important component of any training program.
Normally I recommend that runners do their long runs anywhere from 45 to 90 seconds per mile slower than their marathon pace. Simply do your long runs at a comfortable pace, one that allows you to converse with your training partners, at least during the beginning of the run. Toward the end, you may need to abandon conversation and concentrate on the act of putting one foot in front of the other to finish. However, if you find yourself finishing at a pace significantly slower than your pace in the first few miles, you probably need to start much slower. It's better to run too slow during these long runs, than too fast.