Competitive runners know that the only way that they become faster is to run faster in practice.
At the University of Copenhagen, Danish scientists studied experienced runners who had been running 60 miles a week at a fast pace. One group was told to cut their mileage in half to only 30 miles a week, but to run a series of around 50 to 100 yard dashes as fast as they could. The other group kept on running 60 miles a week at a fast pace. That means that they could run faster for a longer period of time. The runners who did not increase their speed in practice did not improve at all, even though they were running twice as many miles.