Here is the sentence in question:
"...line of performance apparel is perfect for any race, 5k to 50k. "
Is that the proper way to modify race? What is this rule called?
I believe this is just a case of a prepositional phrase in which the preposition is implied, rather than stated outright:
...line of performance apparel is perfect for any race, [from] 5k to 50k.
It's not incredibly formal, but I doubt it would be flagged as an error in most contexts.