When you get home, swing into the driveway and drop off your bag of stuff, then go out for a little while longer.

Alternatively, scout out some longer routes to and from work using bikely, google maps, etc.

My "usual" route is about 5.25 miles one way, but I've got another one that's about 8 miles one way... and if I really wanted to, I could probably hit 15 or so one way.

My commute is generally downhill going to work and the opposite going home... and carrying a heavy pack on my back made it even more tiring. You might check into a rack and panniers for your bike, I've found that it makes the uphill parts a little easier.