Originally Posted by practical

Are you using toe clips or clipless pedals? If so, that will help. Do you have a bike computer so that you always know how fast you're going? If not, that will help. Are you focusing your effort on maintaining a steady and quick cadence (vs. power pedaling)? If not, try that and it will probably help. Do you make sure your tires are fully inflated? Do you wear biking gloves to avoid hand fatigue? Bike shorts to avoid butt fatigue? All of these things help marginally, but marginal improvement in a variety of ways adds up to significant improvements overall. Finally, have you investigated getting a powered bike - one with an electric assist motor? It would make a lot of sense for someone in your situation.