If you speed up, you can finish the race in the same amount of time. Light always finishes the race in the same amount of time from each observer's perspective. It can always speed up because it's massless, but you have mass and there is a limit to how much you can speed up. According to general relativity, an observer outside of the racetrack won't see light change speed, nor will you. But both of you will perceive distortions of space and time to accommodate light's "change in speed" to make it look as if nothing's changed.

Back to the cosmological horizon... We don't see a big jump in the racetrack because the distance between 100m and 200m isn't too massive; likewise, gravity and other forces hold objects together even though space is expanding and pulling them apart.

But there is a certain distance where gravity and other forces can't keep things together. Where that 1000k becomes a 10000k. The pull from one galactic cluster to another is just too weak. Anything past this point is moving away from us. That's why only galaxies close to us aren't redshifting away. But any observer anywhere in the universe would observe that they are at the epicenter of an expansion where everything is moving away from them.