To preserve lean body mass when dieting its important to be sure you are eating adequate protein. Try to aim for at least 1 g of protein per lb of lean body mass to be sure the weight loss is coming from fat and not muscle.

Also, perhaps since losing weight you need a bike that is better adapted to the ligher you. By that I mean when I lost 35 lb I also didn't get as much faster as I was expecting. But my position on the bike, and even gearing (mountain) was for the heavier me. Recently I got a new bike, and new position, with road gearing, and saw a huge improvement.