It depends on how you define fit. Ten miles in 45 minutes is a great start. But I don't measure my workouts/rides in terms of miles. I measure it in terms of duration and quality. If you do a ten mile ride, and a majority of it is downhill, then you're really not getting much of a workout.
I focus on the quality of the ride. I mix things up, mainly because I race.
In terms of weight management, I tend to work on long, slow base miles over the winter months. This helps the body learn to use fat as a source of energy before burning up other valuable energy sourecs. This also prepares the body for more intense workouts as the season progresses. That's when the intensity workouts kick in - things like sprint workouts, intervals (multiple reps at a higher, fixed intensity), hill repeats. This, combined with racing helps me raise and maintain my fitness and speed.
To translate it into something you can use, the basic message here is that whatever you do, increase the distance and/or intensity of your rides SLOWLY - this is to prevent injuries. Keep at it, and you'll find your fitness will improve. You'll find that you can ride farther and with less effort over the same distance as in the past.
Good luck!