There are 2 meanings these days.
The first, more traditional meaning of "pro" would mean you do it "professionally"—as in, that is what you do for one of your primary jobs. I think even someone who runs and has a youtube / social media / sponsorship, and gets enough money to get by, even if they aren't fast, could be considered a "professional runner" under this context.
The more slang version of "pro" means something more along the lines of being very, very good, and isn't necessarily synonymous with getting paid tremendous amounts of money, but has almost entirely everything to do with talent, ability, (speed in this instance, ) and competence. A way to explain how fast a top college athlete is to a younger person would be to say something like "he's a pro".
I thought this was axiomatic, but hey, here you guys are.