I went from a 7 mile to a 17 mile commute (each way) about a year ago. Here is what I found:

. Not enough hours in the day when I ride.
. On a 7 mile commute, once I warmed up, I was warm. Somewhere before 17 miles, though, I can start to get cold again (toes, fingers, etc.)
. Things that didn't discourage cycling at 7 miles each way are more likely to keep me from commuting 17 miles. For instance, I used to dodge thunderstorms and feel pretty safe, but now it's tougher to avoid them.
. Riding 17 miles, then resting at work, then riding 17 miles home 9 hours later takes more out of me than riding 34 miles continuously. I think in that time, my body starts to go into rest & repair mode and then the ride home interrupts it. Doing it again the next morning makes repeats the cycle.
. I found it much easier to stop on the way home for social events or errands with the shorter commute than with the longer one.

Two things that are different between you and me is that you are already commuting 5 days a week. I used to commute 3-4 days a week; that has dropped to about 2 days a week on average. Also, you say you have plenty of time. Time is the single biggest factor that keeps me from commuting. I simply can't get all the stuff I need to get done if I devote 3 hours a day to riding.