I have a 24 mile RT commute, and I'm 24.
YMMV, but I'd sugest the following: 1) eat energy foods while riding, same thing as what you would use if you were riding for 2-5 hours straight. 2) Make two days easy days, and you should feel guilty about the slow pace, remember, it only takes an hour for you to get there at 12 mph. 3) Carb load during each and every dinner, whether it be rice, pasta, or bread, especially Saturday and Sunday nights. This gets the tank Full on the weekends when you get ready for the week. 4) Pick a day, such as Saturday or Sunday, and don't ride at all. Counter intuitive, but not riding for 24 hours can do a lot to helping you out later that week.
flyingcadet