I just started commuting 3 miles one way to work by bicycle a few weeks ago. At 5am in the morning I wouldn't think about walking to work. It's all downhill to work therefore all uphill home so biking only makes sense.
I started with a 20 year old Kona mountain bike outfitted with fenders and slicks. Worked well until I discovered the hubs were wasted, the cassette was toast, the chain was stretched, etc. I have it dissassembled in the garage to repair now. I did get enough rides into to firmly convince myself I am a dedicated bike commuter now, and that made it easier for me to drop the cash on a brand new Specialized Rockhopper Comp. I found that by lengthening my commute by about a mile I can travel by trails 1/2 of the trip. I far prefer mountain biking to road biking and with the new bike it's an absolute blast. Good luck on getting started with your commute by bike! You'll get hooked!