INTENSITY MAKES YOU MORE FIT
Gabe Mirkin, M.D.
If you jog regularly and want to be able to run faster, run faster than you usually do a couple of times a week, even though it may increase your chance of injuring yourself and you may not be able to run as many miles.
At the University of Copenhagen, Danish scientists divided experienced runners into two groups. One group cut their mileage in half to 30 miles a week, but ran a series of 50 to 100 yard dashes as fast as they could. The other group kept on running 60 miles a week at a fast pace. The runners who ran fewer miles faster had a seven percent improvement in their body's maximal ability to take in and use oxygen. 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. This doesn't mean that you should go out and try to run fast every day. Intense exercise damages muscles and it takes at least 48 hours for muscles to heal. Try to run faster perhaps one or twice a week, never on consecutive days. Never run fast when your legs feel heavy or hurt.