For a healthy fast food you can't beat a decent sandwich you know! Use some good bread rolls, pitta pocket etc and chuck in some already-prepared salad, houmus, feta cheese, olives ... you get the idea! Very filling and much better than processed stodge! And if you are really on the run, carry some bananas, grapes, cherries or even some dried fruit like dates and figs in a little sealed container. And drink lots of water - about 2-3 litres a day. Dehydration can make you feel very tired, too.