My personal experience has been that I feel better if I eat a little breakfast before I leave, snacks throughout the day, then a bigger meal an hour or two before I head home.

(I have a high metabolism, by my scale, a "little breakfast" is two packets of oatmeal with extra raisins, with the amount you're eating maybe just a banana. My bigger meal is usually something like whole wheat pasta salad with veggies.)

What are you drinking when you ride and with your meals? Once the weather got really hot here I found if I didn't replace electrolytes while I was riding I'd slow down and get a headache, now I mix water and gatorade and feel much better. Also, a glass of soy milk or milk with your after-ride snacks will add some protein.

Edit: I forgot the main reason I wanted to reply. If you can continue your 30mi RT commute feeling so lethargic you're goin 3.5mph, that's serious mental toughness!! Good for you, keep up the fight, soon your commute will be one of the more pleasant experiences of your day.