How much more fit is "Ultramarathon man" Dean Karnazes than you?

This one blew me away when I heard it on an archived Competitor Radio podcast (which I listened to while running, appropriately enough).  When Dean Karnazes, author of Ultramarathon Man and all-around ultraendurance guru, was training for the North Face Endurance 50 (running 50 marathons in 50 states in 50 consecutive days, which he completed in one piece),  he worked with Chris Carmichael, Lance Armstrong's coach, to fine-tune his training.

In addition to using 100-mile ultramarathons like The Western States Endurance Run as mere training runs (!), Dean and Chris established a benchmark that let them know when he was ready for the Endurance 50:

>> Complete a sub-four hour marathon with an average heart rate of 110.

Read that last part again -- an average heart rate of 110!  Most people hit that getting off the couch; try running even a mile at that HR and you'll know what I mean.

Wow.  Double wow.