You don't get stronger when training, you get stronger when resting. When you train you put load on your muscles & do small amounts of damage. In response to this they adapt to the loads when you let them recover, they do this by rebuilding them selves stronger in the way you load them. 10 miles/day is more than enough to produce over-training if the rider does't rest. Get close to maximal power for shorter durations every day & you soon start with compound damage to your muscles. This results in reduced power output & you simply can't ride as fast as you used to.
Power breeds economy, and high cadence is economical.
Higher cadence is more effective long term as it reduces recovery times & so allows you to produce more power for longer. Lower cadence is the most economical use of oxygen & ATP.