Because I've never been a runner, really, I have always held up the marathon as one of the ultimate physical challenges one could imagine doing. I've never really put any brain power into it, simply because the thought of running so long never interested me in the past, and would be pretty difficult now, given that my knee gives me a hard time just walking downtown.
Two minutes of internet research brought me to this handy calorie-burning calculator for runners and, after pumping in my imaginary numbers, was given around 3000 calories burned for a full 26 miles. I burned more than that at the 136 km St. Tropez race this year and I'm pretty sure I remember passing 5000 on the 2012 Etape du Tour in the Pyrenees. Who knows what Haute Route was like, since my computer was broken most of the time. Maybe Rob can fill in those blanks.
But I don't even know if calories tell the whole story. I can guess that running a marathon involves a consistent effort over the whole time you're on the road, for example. For most cycling events I do there are lots of long descents that allow you to recover. Conversely, I'll bet there are few, if any, times during a marathon that you are forced into the red as much as you would be on the bike during a long climb, bridging a gap, or just trying to hang onto a pace line. Maybe it's an unfair comparison.
Have any of you run a marathon and done a big mountain sportive/gran fondo? If so, enlighten me. I'll never know it from my own experience!