Normally I recommend that runners do their long runs anywhere from 30 to 90 or more seconds per mile slower than their marathon pace. This is very important. 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. Save your fast running for the marathon itself. There are plenty of days during the rest of the week, when you can run race pace.