I have just started riding religiously and I used to ride a 43 pound mtb from Wal-mart (yea, lol). A friend of mine is training for a tri so he bought a specialized allez. I couldn't keep up naturally so I bought a Trek 1200.
My speed avg and time increased drastically (over 3mph avg) from the mtb to the 1200. It's also much more fun to ride the Trek. Here's the results:
About 4 days apart, 15 mile ride, same route, same conditions:
MTB = 13.7mph avg
Trek
1200 = 16.8mph avg
First 8 miles:
mtb = 15mph avg
Trek = 17mph avg
Trek Cycling Log
Just fyi, I'm 31yrs old, about 190 lbs (shooting for 170 maybe)
I used to ride my wal-mart mtb bike every now and then a couple times a week about 5 miles at a time. Then I would stop for like 6 months, a year, etc. So never really persistently. Since the Trek I plan on building up my miles and speed each week until I'm riding maybe 100 - 150 miles 5 days a week. To loose weight and get in physical and cardio shape.