I agree; just because it's a shorter ride doesn't mean it's easier. I started riding to work (8.5 mi each way) three times a week. It took a while to get used to that. After my body adjusted, I was able to take longer detours in to work for a 15-20 mile ride in. It took a couple of months before I had the energy to do go out for a weekend ride. But eventually, the weekend rides got longer and I would do 25-35 mile ride in to work (I can take half days :-). Don't forget that you need proper fuel after your commuting rides. Perhaps you should try three days a week (MWF will give you recovery days) so that your body can get some rest until it adjusts to the new riding regimen.