I suppose you could be putting on muscle, but I'd be really surprised if that explains all 20 pounds of your weight gain. What changes do you otherwise see in your body? Are your legs bigger and more defined or do you see additional fat in your stomach, hips, thighs, and arms? If you feel trim and toned, then I wouldn't worry too much about it.

Are you sure you're not overcompensating for the mileage in terms of what you're eating? Ten miles, while fantastic, isn't a long distance from a caloric expenditure point of view. Age, too, can change your metabolism. If you'd like to lose a bit of the weight, I'd encourage you to increase both the amount of time you're spending on the bike and the intensity of your rides.