I wouldn't call Greg Lemond morbidly obese. He's overweight, yes. The guy in the green shirt riding on wheels far, far narrower than he should be, now that guy is morbidly obese. But I'll give him this. At the moment that photo was taken, that fat dude was on a ****** bike, and not sitting on a couch chowing pizza and beer like a lot of other people. Hopefully he keeps it up, and burns off some of that fat.

Btw, I did a sprint triathlon when I was around 374 lbs and actually beat like two people. Kinda funny.

As far as eating goes, I've got lots of experience trying to lose weight while also trying to cycle long and hard (for me). Shorter distances (like up to 32-35 miles, say) can be done fairly fast, up to around my tempo speed. At real tempo that can be pushing it, depending on my overall energy state. Further can be done on bodyfat alone but not fast. It's all about knowing how much energy my body can provide on bodyfat alone. That's going to be different for each person. I know that if I'm in zone 2 I can provide most of the energy on burnt fat, and if I'm significantly into zone 3 it's also having to tap either glycogen stores or else externally provided sugar. How long that can go on depends on the state of the glycogen stores. Since I have no way of measuring that, all I've got to go on is past experience, including the several times I've full-on bonked on long, fast rides. Having bonked before, I can generally watch out and see the signs before it gets that far. If I don't carry any calories on my bike for a long ride I try to plan routes that never take me more than a few miles from a gas station, and carry my wallet with me.

There's no shame in having to stop and eat something on a 45 or 50 mile ride or longer. When you bonk, it's not a matter of willpower. All the willpower, grit, stubbornness, strength of character, or whatever else isn't going to make up for the fact that your brain is dying due to lack of fuel, and shutting your body down to keep that from happening.

That said, I fully believe that it's possible to train one's body to be better at metabolizing fat, so that one can ride harder/faster on internal stores alone.