But are the paths straight or twisty with a lot of dips and climbs. If the latter is the case then the path is whats really slowing you down. I know this because I've tried bike paths before because I wanted to avoid traffic but ended up with an average speed of only around 12 to 13 mph because of all the slowing down I had to do, and having to keep changing gears. Once I started using the roads I was able to increase my cruising speed to 17-20 mph with an average speed of 15-16 mph. Cruising speed is something that is hard to achieve in trails.

Also I have the same bike as you do. Before the Fastback I used an '84 Centurion Accordo RS, 28 pound, 12 speed, steel road bike. I still interchange the Fastback and the Centurion every week and I can tell you that although accelerations and climbing are easier on the Fastback I noticed that the amount of work I do to achieve the same cruising speeds is about the same.

You may also want to start using the bigger ring, 52T, on your Fastback. It makes a big difference in speed and amount of spinning. I normally use 52/19 or 52/17 when on the flats and small climbs. The only time I drop to the 42 is when I'm climbing overpasses and pedestrian overhead walkways over the the highways.