I picked up a pos mountain bike from my dad, spent 75 bucks getting it riding ready, bought a bunch of gear, and have been going for nice (cold) rides about once or twice a week. I rode in to my work one day (about 20 kms) on the sidewalks the whole way, and that was fine, but on the way home, I ran out of leg. I actually had to get off the bike and walk a few times to rest my legs. It was probably the worst ride of my life.
Last summer, my girlfriend was borrowing my car for her job most days, so I ended up riding in to work about 3 of the 5 days. It was excellent for me, and I intend to do at least that much riding this summer as well. My goal is by next winter to be in good enough shape to ride to work every day. Maybe save myself a few bucks in gas, and get some intense biker legs as an added bonus.