I've always been curious, do Hobbits have crazy high metabolism or extremely poor digestive systems? All those calories must be going somewhere, right?
I assume it's a crazy high metabolism. Smaller creatures generally need to eat proportionally more than their larger counterparts. Taken to an extreme, some small birds need to consume their own body weight in food every day.
They like keeping busy! Seriously though you're presumably already a pretty active person if you can one day just decide to go for a hike to the other side of the world.
if you can one day just decide to go for a hike to the other side of the world
They didn't do it in one shot though. They went to Fatty Bolger's place first, then to Tom Bombadil's, Bree, and Rivendell. I imagine that they gradually improved their strength and stamina with each leg of the journey. Frodo's initial task was just to get The Ring to Rivendell, after all.