I started out late in April with a new bike Purchase in roughly the same situation. I hadn't touched a bike in many years, am 27 years old, 5'10" and 215lbs. I went with a road bike, and the smaller tires and lighter frame might be helping out a bit but I started at roughly 10 miles and 14.5 mph. I commute to work whenever possible. 3 Months later I averaged 17.5mph during a Charity 50 mile ride in the rain with 2000 ft of ascending. My commute to work (3.85 miles) has gone from 13:30 down to 11:30. You'll see improvements as long as you keep with it. It will come quicker than you think.