'I could endure a few more days of ultimate stress' - UVA Supplement

I looked carefully at the sweat-drenched runners pass by, as they entered their next grueling lap across the soccer field. Their physically spent faces in totally agony told me, "If you start this sport, then you will regret every moment of it". Despite the blaring signals given by the older kid's expression, I, the naïve eighth grader, enthusiastically signed up for cross-country.

The decision was rash and rather shortsighted, considering I had a tendency to rush into situations. For this mistake, I paid dearly. Every weekday, I ran in irregular shaped figures over and over again until my head went completely numb and until my muscles screamed out in pain. Sometimes, I had the privilege of viewing my afternoon lunch with other vibrant colors mixed in a repulsive slime. But, everyday was a continuous struggle, as I, the jedi knight, tried to balance the forces of sports and education. Eventually, the external pressures started to build up and the stress pushed me toward the edge of spontaneously combusting.

Although I often commented how I hated cross-country with a passion of a thousand suns and how if cross-country was a tangible thing then I would murder it in its sleep, inwardly, I appreciated its subtle benefits. When pressured by a bunch of domineering calculus problems or a crumbling relationship, I usually wanted to escape the reality of a situation and would often "go underground". A safe zone or a relief hideout. Jogging a few miles had this magical effect on me. I shut myself out from the cacophonous noises of the world and immersed myself in only controlling the gentle, rapid sound of my breath and listening to the steady pulse of my heart. Mentally, not physically, renewed, I returned to the surface and could endure a few more days of ultimate stress.

Please give me some good ideas, and please fix some grammatical mistakes...

I looked carefully at the sweat-drenched runners pass by, as they entered their next grueling lap across the soccer field.

you don't need the comma

Their physically spent faces in totally agony told me, "If you start this sport, then you will regret every moment of it". Despite the blaring signals given by the older kid's expression, I, the naïve eighth grader, enthusiastically signed up for cross-country.

I would pick either 'Their physically spent faces' or 'Their faces in total agony'
You went from (plural) their faces to singular (kid's expression)

Sometimes, I had the privilege of viewing my afternoon lunch a second time with other vibrant colors mixed in a repulsive slime.

When I first read it I was thinking... ew why would he eat a slimy colorful lunch?

and would often "go underground". A safe zone or a relief hideout.

I would change this, and get rid of the go underground part because it didn't really flow, like you were quoting a movie I hadn't seen.

Overall I really like this essay. The part about how murdering it in its sleep actually made me laugh out loud (I feel the same way about swimming) I like your humorous style and how you really captured the love/hate aspect of a sport like this well