I haven't raced Iron Bike, although I sort of fancy it. I also know Matt and I have differing opinions on what a race is, but hey, here are some races I'd say were harder than Iron Bike. All these races are non-stop. You snooze, you lose.
-The Iditarod Trail Invitational is, most certainly, a race. 1000 miles, ~14 days minimum at in super low temperatures (say -20C down), mainly on your own.
-Colorado Trail Race, 500 miles and 65,000'/20,000 metres climbing, 90% singletrack, unsupported. Starts a mile up, and keeps going. Record is sub 4 days.
-Arizona Trail Race, not sure on the stats but its a 750 mile unsupported race on singletrack. Plus you have to walk throught the Grand Canyon.
-Highland Trail Race. 550 miles. In Scotland. Bring on the weather. And the chafing.