Despite how silly it might seem, unless otherwise specified, speedsters run like everyone else. But all speedsters are not created equally so each speedster, depending on the mechanics of their super-speed, might leave a different footprint or spread of footprints depending on how their powers function and how how they are based in their comic physics.
This means the Flash and any other speedster unless their power remarks otherwise is running in the same way as any other person, one foot after another and their spacing for each footfall would be the same distance apart as any other runner. (Which begs to be asked, why don't their hip joints explode into flame after a few thousand steps at this incredible speed?)
To help visualize how this mechanic might work, we will borrow a page from superhero roleplaying games such as Hero Games Champions imprint, which uses a variety of game mechanics called "limitations, advantages or special effects" (the description of the power in question that might distinguish it from others with a similar power) that determine how that running is perceived by others.
This means you could have two players who are using super-speed in the game but each defines how that super-speed functions during the course of play and what that mechanic might look like.
For example: the Flash runs up walls because his power description says he is so fast he can defy gravity for a time. In Champions, this power limitation special effect is called "Flight while touching a surface." Other speedsters may have different "special effects" which might change the way their footprints appear on the ground.
However, the special effect of that running may prevent their footprint from being perceived in the same fashion. Some descriptions of other speedsters and how their powers are perceived (in their respective universes) and what their footprints might look like.
All of DC Comics Flashes powers include a temporal element that allows them to move quickly between two locations with a movement that resembles running, so to view their foot-falls it would appear to be the movement of a person who is moving very fast between two locations as a runner would. But their powers allow them to move without disturbing much of the environment around them unless they wanted to, so they might leave behind a foot-fall that would appear to be feather-light due to the nature of their power's conservation of energy and entropy. Even though any Flash can move faster than the speed of sound, he or she will not create sonic disturbances unless the boom and resulting over-pressure wave is a desired effect.
DC Comics, Superman, on the other hand, while capable of using super-speed, would not be able to claim the energy conservation capabilities of the Flashes and would leave deep footprints since part of his power would be derived from his application of his superhuman strength, reaction time and reflexes. He might also have a larger stride as well, a reflection of his strength being applied to his running.
Marvel Comics, Quicksilver and Speed Demon have dense, superstrong muscles, a denser than human bone structure and super-lubricated joints. They are both fast and strong, so they leave a deep footprint but would not be hindered by the sand because of their incredible strength. They may also have a slightly larger stride than an normal human their size.
Marvel Comics, Northstar is a mutant speedster capable of actual flight. If he were running, his footprints might have extremely long gaps between each footfall if he were partially running and flying, simultaneously.
Marvel Comics, Triathlon is known to be able to reach speeds of a hundred miles an hour, but his running is faster due to his extended stride which is a function of his superhuman strength and increased reflexes.
