How do you run fast?

Longer is stronger, in a manner of speaking:

When non-athletes want to run fast they set about moving their legs as fast as they can, so you might assume that Bolt has achieved this exceptional record by making his legs move faster than everyone else has. But this idea doesn't stand up - in fact, it falls flat on its face.
"Elite sprinters don't swing their legs any faster than recreational runners," says Dr Sam Allen of Loughborough University.
Instead, the difference is that a top sprinter takes longer and more powerful strides.
Research shows that an amateur runner often takes between 50 and 55 steps to complete 100m, while an elite sprinter takes in the region of 45.
"Elite athletes generate so much more power, owing to the fact they naturally have more fast-twitch muscle fibres. These elite athletes therefore spend a lot less time on the ground which results in them being propelled forward much quicker," says Allen.

Studies led by the American based researcher Peter Weyand have found that at top speed an elite sprinter's foot will typically spend 0.08 seconds in contact with the ground at the beginning of each stride compared with about 0.12 seconds for an amateur athlete.
Sam Allen says the fastest sprinters seem to be spend about 60% of the time in the air, with no foot on the ground, while for amateur athletes it's more like 50%.