Boy, this strikes me as a difficult medium to tell this story in. On TV or in film, you have more of a visual spectacle as cars race around everywhere, and you have a better sense of the passage of time, which is nice to have in a race. In a novel or short story, it's easier to skip over uneventful parts. Comics is almost the worst of all worlds here, but maybe I just don't have enough imagination. Still, many stories about races have been told in comics; Speed Racer was originally a manga. Certainly, you should have a look at it or other such comics to get a sense of how others have tackled the problem.
Anyway, when constructing a plot in any medium, you need to know what your "beats" are: the events that make things progress. Usually, everything in your story should either be a beat or help you progress from one beat to the next. You can construct a plot outline to find your beats (each item is a beat):
1. <PERSON> gives his cousin <PERSON> a magic ring as a gift.
2. 805
But <PERSON> learns from <PERSON> that the ring is dangerous and must be destroyed in the fires of Mount Doom.
3. Therefore, <PERSON>, <PERSON>, and some friends set out from the Shire to Bree.
4. But they get attacked by ringwraiths on the way.
...and so on.
I learned a technique just yesterday for constructing a plot, and I learned it from what seemed to me an unlikely source: this video with <PERSON> and <PERSON>. Their advice here applies to all storytelling, not just South Park's style. The tl;dw of the video is you're in trouble if you use the phrase "and then" when connecting your beats; instead, you should try to use "therefore" or "but" (as I did in my Lord of the Rings example). I wouldn't take this advice too literally; the important thing is to understand how the beats are connected beyond merely happening in sequence. I've also found a nice article that expands on this.
Also, a subplot can give you something to cut to when nothing interesting is happening. If you don't know what to follow a scene with, you can cut to something else, then cut back once your characters are back on track. For instance, a fight might break out in the stands, or something else might be happening in a completely different part of town. What else is going on during your race? 569