Avoid crowded races. Most runners in the large population events run more than three-quarters of a mile farther than marathon distance. Since you don't get credit for the extra distance, it is harder to run goal pace in these venues.
Look for smaller marathons that have a good record for producing fast times. Visit www.MarathonGuide.com and www.RunnersWorld.com for a lot of information with runner reviews, which are usually quite revealing.
Courses that have slight upgrades of short duration allow for a varied use of muscle groups, which often helps in reducing muscle fatigue. There are many downhill courses that can help speed you up – if you train on downhill courses.
While you cannot control the weather, look for venues that tend to have high temperatures of @ 60F (14C) or cooler during the month when you are running your race.
From Jeff's Galloway's Marathon FAQ
Jeff, Thank you so much for writing this response. It’s so inspiring to be running under your program (and having met you last year).
I’ve run my first two marathons in over 6 hours, my training runs now suggest I will complete my third one next month in over 6 hours as well.
Would I like to run a sub-5 hour, qualify for the Boston Marathon someday? Sure thing, right now though, I’m running without injuries, enjoying myself and getting stronger daily. Thanks for sticking up for us slower athletes (and I think anyone who finishes a half or full marathon can consider themselves an amateur athlete) and letting us know that in your experience very few runners have it out for us “slowskis”.
– Danielle Stolman