If a cross country runner is out there I also want to know how to say what is called fartlek. A fast up tempo speed run followed by recovery running. It's interval training but no walking is allowed. Also is there a particular word for a long distance run and cross-country running (as in the sport name)? Also what is a side cramp, or side-stitch called, the pain in the side of your abdomen which is likely caused by lack of water, potassium or weak abdomen?
For long distance run, the sports tech term you're looking for is Carrera de fondo. Actually, any sports endeavor that rewards endurance over speed is said to be about "fondo" físico, literally "bottom" or "deep end" probably an allegory about finding one's physical endurance depth.
Anything cross country (running, walking, etc) is said to be Campo-traviesa
As for fartleks, the English loan word "Sprint" will be well understood in Mexico with your intended meaning.
As for the side cramp, it's a funny idiom: "Dolor de caballo" or "caballazo" refers to that kind of pain, probably referring to the Broken stride when you get hit by that pain while running, that resembles a horse's stride.
For example: "No pude terminar la carrera de fondo a Campo-traviesa, pues al terminar el primer sprint me dió un insoportable dolor de caballo"
For "fartlek" we use the same word: "fartlek". See https://es.wikipedia.org/wiki/Fartlek:
El Fartlek —«juego de velocidad» en sueco1 — es un sistema de entrenamiento que consiste en hacer varios ejercicios, tanto aeróbicos como anaeróbicos, principalmente ejercicios de carrera, caracterizados por los cambios de ritmo realizados a intervalos.
However, as far as I know, and unlike the other words you ask for, this is generally not used outside the running community, and therefore generally unknown.