I loved Decathlon, Summer Games, ...
All games are basically the same, only some changes in the animation (long jump, hurdle, ...).
I would start with the running system and a varSpeed variable.
- Keys: after left only right increases varSpeed (after right the left must be pressed)
- every 0.x seconds varSpeed decreases (player gets slower)
That's basically the whole system - of course there must be a build up in speed ( you can't just go to 100% speed at once)
depending on the varSpeed you make the animation of the player slower or faster.