I'm no expert on nutrition, but my recommendation would be to have a good breakfast before heading out (I would opt for oatmeal personally) and carry some healthy snack for when you get to work (banana, yogurt, fruit) and a good lunch (sandwich, fruit etc) and another decent sized snack to get you home. I used to commute home once a week and that worked for me. A good breakfast and lunch and then before heading home I'd have a banana and a granola bar and I'd stop half way home for another granola bar. Then I'd have a hearty dinner when I got in (lots of vegetables and carbs). Seemed to work for me and my commute home was 40 miles. Whenever I've done longer riding, the most important thing to keep me going through the day has been breakfast. A good bowl of oatmeal with dried fruit seems to give me enough energy to keep me going.