You do not want to begin speedwork in a marathon program if you have not done it before. For an introduction to speedwork, check the Spring Training section on this web site. Save your energy and concentrate on quality runs the rest of the week. Consistency is most important. 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. This is very important, particularly for Advanced runners who do speedwork during the week. The physiological benefits kick in around 90-120 minutes, no matter how fast you run. You'll burn a few calories and trigger glycogen regenesis, teaching your muscles to conserve fuel. Running too fast defeats this purpose and may unnecessarily tear down your muscles, compromising not only your midweek workouts, but the following week's long run.