I commute on a touring bike with rear panniers, and average about 15 mph for 16+ miles each way. I don't push too hard, so that I don't have to take rest days. I only started commuting last summer, and ride about 4 days per week. I tend to take Fridays off, so that I can be more rested for the weekend group rides or brevets. I have a 200K ride coming next weekend, so won't be pushing very hard next week.