The shortest route I can take to work is 19.3 miles, generally I take routes that are 20 - 21. When I first got my bike I never thought I'd be able to ride to and from work in one day. My friend at work, who happens to live a couple of blocks from me, suggested we try riding home one day together, just so I could get comfortable with the route (the mile surrounding work is really difficult in terms of a myriad of bike paths and crazy intersections, its very confusing) and the idea of commuting in general. This helped immensely. I drove into work, we rode home, then I took the bus into work the next day. I continued to do this about 2 days a week for a while at the beginning of the summer when it was still too cold to ride in the mornings (I'm slowly beefing up my cold weather riding wardrobe so hopefully this won't be a problem next year). Then when it was warm enough and I knew I could handle riding 40 miles in one day I tried for the round trip commute. I am so not a morning person, and it was really difficult for me to get up, get dressed, and get on my bike. I would try to get to work before 8 so I could make it down to the showers in the basement without everyone seeing me all sweaty in my bike clothes, but I would say nearly half of the people I work with also cycle, so its not that big of a deal. After showering, this usually meant I was at my desk a half hour later than usual, which makes sense. It takes me about 1 hr 20 min to ride to work, it takes about 45 minutes to drive (the traffic is terrible), but I shower quicker at work. On the way home the traffic is even worse, it can take an hour to get home in the car, and it only takes 1 hr 5 min to ride home because its almost all downhill. There's this one particular intersection on the highway (if I take that route) where the cars always back up. You get to pass about 1.5 miles worth of cars flying by on your bike. Granted they pass you again after the intersection, but its still fun to not be the one sitting in the car being frustrated.
So say I usually spend 1.5 hours a day driving to and from work, then an additional 1.5 hours at the gym and getting home, that's 3 hours for commuting + exercise (not to mention roughly $6 in gas at $3/gallon, and x volume car emissions I'm putting into the air). Contrast that with 2.5 hours on the bike for commuting + exercise in one day, I've saved time, money, and the environment.
I have a fairly flexible work schedule, I just need to work 40 hours in a week, and no more than 10 hours in one day, so I can get in a little late one day and make it up another day. I actually get home earlier on the bike than if I drive to work and then go to the gym. The only thing I'd like to add is it might be a good idea of having back up transportation in case of rain or illness or some such thing. Its easy (although time consuming) for me to take the bus, I work with two people who live near me I can bum rides off of, and if my husband isn't traveling for work, he can come pick me up if need be.
All that said, I am done commuting for the season. It doesn't get light enough to ride until 7:30 am, and it gets too dark to ride without lights at about 5 pm. Daylight savings time will make it even worse in the evening. There is no way I would ride on that highway in the dark, its dangerous enough in the light. I prefer the back roads which are great in daylight, but would be treacherous in the dark. I missed the shoulder season where I could have gone back to just riding home, but I've been on travel and vacation. I miss commuting already. This is probably way more information than you wanted, but I'm in a rambling mood . . . Good luck, give it a try, you just might like it. Oh, and lots of people agree on this, that I seem to have more energy during the day after having ridden in.
The best part about going up hills is riding back down!