It might be useful to understand a bit more about what happens to your body when you are on your bike. Mrs M is on target to hit 4000km this year, her best ever since she got into cycling about ten years ago. Her mileage is amazing, way more than mine, and it's obviously doing her some good but I know that she isn't making the most of her time on the bike.
The basics are (from memory), if you want to get fitter you need to spend a minimum of one hour a week at 75% of your maximium heart rate. The body reacts differently to different levels of exercise because it responds to the expectations placed on it. For example if you push a high gear (low RPM, slow, hard pedalling) your body will react by increasing the bulk of the muscles being used, like doing weights. If you use lower gears (high RPM, fast pedalling) your body will respond by improving your cardiovascular system in an effort to provide your muscles with more oxygen. Somewhere between these two extremes is the zone where the body burns fat, but you can't burn fat effectively without a good level of cardio fitness. Aim for good cardio and everything else will fall into place; improved fitness, better muscle tone and reduced bodyfat.
Many people climb on their bikes and pedal at what seems to them to be a comfortable rate but the problem is that as pi-peds we are self programmed to use our legs at a walking pace, as a result they never get into a heart rate zone which provides any substantial benefit to their fitness levels. Too many folks trundle around in a gear which is too hard and whilst this level of exercise is better than no exercise at all it makes sense to try and optimize your on the bike time for maximum effect.
Short of buying a heart rate monitor...
At least three times a week you need to spend at least twenty minutes riding hard with your legs spinning like the clappers. Aim to get out of breath. Add ten minutes warm up and ten minutes warm down at the beginning and end of each ride where you pedal lightly at relatively low speed.
The self-gratification artist doesn't deserve you.