Your body is adapting to the exercise requirements of your current bicycle and the route your taking. If your just into it for the exercise and don't care to add miles simply put some aggressive tires on your bike with lots of rolling resistance. Another option is to change your route and add hills. Adding weight and hills is another option. If you're riding a road bike pickup an old mountain bike with aggressive tires. Big box retailers have very cheap bicycles, which will fulfill your "lead sled" exercise requirements! The cheaper the heavier the fewer miles you will be required to pedal for your exercise requirements. Just remember to replace the cheap tubes in the tires and replace them with good quality tubes and new rim tape from your LBS so you wont have flat tires about 5 miles from home. Thrift stores are also an option but you must be careful about the condition of a used thrift store bike. You can easily end up with more money in a $5 thrift store bike than a new Wally World $79 bike!