In reply to EeeByGum:
Are you carrying much extra weight (i.e. have you any fat/energy reserves?) and are you racing there and back? what sort of time does it take you? are you on or off road?
If you have a heart rate monitor try keeping your heart rate in the 65-75% range and try not to go higher (you may need to on hills) this should help improve your efficiency and reduce your energy consumption a bit
I do about 9 miles each way - generally in about 30 minutes depending on getting held up in traffic
I leave in the morning after a cup of tea, no breakie - eat breakfast at work - normally a bowl of porridge or some cornflakes
bagette sandwich for lunch (eat half at lunch and eat the other half about 3pm
Ride back about 5-5:30 (tend to be a bit gentler on the way home)
Eat a decent evening meal (tends to include quite a lot of carbs - bread, pasta etc)
Having been road riding since August last year and being a smoker carrying about a sotne of extra weight I am not the fittest in the world at the moment (commuting is helping to change this) and I am reaonably happy doing this every day when I can... it's more a case of if I can get up early to do ride than the actually riding
Have you looked closely at your route? plot it on:
http://www.bikehike.co.uk/mapview.php and tick the show elevation dat box on the map control panel, you may find you are going downhill on the way in and up hill on the way home which may be distorting the way you burn energy.
Maybe try eating a small pasta or something carby rather than sugary around 3?