Ride your planned commute route on a non-workday first. Time your ride. If you're planning on bicycle commuting every day, ride your route again the next day. If you can. Given your characterization of how little you've ridden your bike, you may not be able to handle cycling 20 miles a day on back-to-back days, much less 5 days in a row.

Don't force yourself to do more than you feel comfortable handling, or you'll likely give up. If you need to work up to 20-miles-a-day full-time commuting, you could start doing something like going for 5-mile rides every day before you head to work.

What it comes down to in the end, though, is you MAKING commuting by bicycle a priority in your life.