I should probably go to a running forum, but you guys might get where I'm coming from with this cycling analogy. Most people seem to agree that if you can ride 70-75 miles in a relatively comfortable fashion, then you should be able to ride a century without much trouble. What would be the equivalent for a marathon? How many miles should I comfortably be able to run to be pretty sure I could do a marathon?

I would hope to finish somewhere between 3 and 3.5 hours, so does something like 2.5 hours make sense, or should it be higher?