While true, the energy expense doesn't really scale with speed, so if a couch potato got up and decided to do 100k in, say, 8 hours, that's around the same amount of energy.
NB nobody would be able to do this without ingesting a substantial amount of food during. If you didn't start eating hourly after about 1-2 hours in, you'll "bonk" or run out of glycogen.
It really depends on how fat adapted your metabolism is. When untrained people exert themselves their muscles tend to produce most energy from glucose and very little from fat. By doing a lot of zone 1-2 training you can gradually shift your metabolism to rely more on fat, at least at lower effort levels where you're not limited by oxygen. This allows you to go longer than 2 hours without bonking.
