A walking human gets about 75 MPG (miles of travel per gallon of food energy). A biking human? Even better, at 290 MPG.
But when you factor in all the fossil fuels that go into making that food, we only get 18 MPG walking and 70 MPG on a bike. Check out the full rundown of the energetic calculations at Do The Math.
I assume they base this on the average too-much-meat-eaters diet? All the more reason to get some (environmentally cheaper) protein variety in your diet :)