In the summer I'm doing a charity ride - 160 miles. It's a long way but I've done 150 before so it's not impossible even for me.
A friend compared it to a marathon which got me thinking.
When I run at 5mph for an hour I burn about 1200 calories according to my HRM. So a Marathon would take me 5 hours or 6000 calories (assuming I could keep going at 5mph).
Cycling (between 12 - 16 mph) burns about the same per hour. However cycling 160 miles is going to take at least 10 hours at 16mph so that's at least 12,000 (again, assuming I can keep going at this speed).
So, from a simple energy expended in calories point of view, this bike ride is going to be as tough as running back to back marathons. It may not be so tough on my joints, but it's going to take some energy.
Maybe I need to give this more respect than I have been. I'd be training like a crazy man if I were doing a double marathon on June!