It's a tell-tale smell. Well known and documented. Pliny mentioned it not so long ago as a mean to measure when he's done enough cardio
Personally, I never done enough cardio to reach this state, nor do I exercise without energy readily available. Even when I was jogging long distance (before weight lifting) I'd have a 1000 cal breakfast and hop on the trail 5 min later. It upset a lot of people to train with a full tummy, but it's like everything, it can be trained. Jogging for 3 or more hours requires a LOT of calories. I'd take 1l bottle with water mixed with energy powder (~400cal) and a couple cereal bars on top to eat 1/2 way