Can I Run a Slow Marathon?

A friend at work wants to bet me that I can't run SLOWER than a 3:40 marathon this weekend at the Illinois Marathon. What? You want to bet that I can't run slowly? Isn't that a gimme? I can run as slow as I want. Or can I?  My stated plan is to churn out 9:00 miles until the 20 mile mark, then go as I feel--faster if good, slower if bad. I plan on breaking 4:00. Probably 3:55. That's SLOWER than 3:40!  I guess I should take this bet. Why would anyone bet me that I can't run a slow marathon?

Joe, the guy from work, thinks I'm too competitive and in too good of shape to run 9:00 miles. He thinks I'm capable of a Boston Qualifying time...or at least MUCH faster than my "break 4 hours" plan. When people pass me, he doesn't think I can hold back. Sure, I can likely run around 3:45 if I push it and have a good day. Maybe even faster. But I don't want to risk injury. I've been running really well lately. I can knock off 8-minute miles with a 60% heart rate reserve average. And I can run around 7-minute pace at 75% HRR.  Everything slower than 7:30 pace seems easy...but only on short runs. I have the endurance to run 30 miles fairly easily. I can run 10 miles fast. Unfortunately, I don't have the stamina to run 26.2 at a reasonable pace (7:45-8:15). Joe's wrong. I do have the discipline to run under control and steady. Maybe I can run a 3:40 or better this Saturday. But I won't try. Joe, still want that bet?

I appreciate the confidence my co-worker is placing in my running. Still, I guarantee I'll run slower than 3:40 this weekend.

UPDATE: Finished in 3:47. Smooth and easy. Even saw Joe twice on the course cheering me on! Thanks. Did I win our bet?!