What would be a fast time to run a marathon in?

I really didn't see a lot of value in the previous answer that "if you want to walk, don't waste your money". There are 34 million runners in the US, but 38 million walkers and there are a lot of people who do races for health and travel. Some even do it for speed (and finish faster than a lot of the ones who run).

Someone else on Answers.com suggested that an average marathon time for men is 4.5 hours and women is 5 hours, so anything under that would be good.

Based on scaled comparisons for walkers vs runners on the world records, that would translate to a 6:20 marathon for men or 7:20 for women. There are a lot of walker friendly races like the Mayor's Marathon in Anchorage or the Honolulu Marathon (with 9 hour limits or greater).