The amount of fat you have is fairly irrelavent, since it's so energy-dense, even someone with a small absolute lbs of fat still theoretically has enough energy packed away for many many miles of activity.
The answers are probably twofold, one, endurance activities are pretty specialized. An experienced runner uses less energy (or, is more mechanically efficient) than a non-experienced runner, even if that person is a experienced biker, or rower, or jump roper or whatever. For instance, the greatest biker in the world, Lance Armstrong, finished almost 900th in the Boston marathon after basically being the shit at biking for like 10 years.
Second, biking is a non weight-bearing activity, as opposed to running. So having bigger stronger muscles helps more than in running for long distances. And enduroweenies usually have extremely low muscle mass. I'm sure you could also rep out 20 lbs on the leg extension machine many more times than your runner friends too.