You have to use at, especially because if you are talking about passing a 15-meter mark that's on the ground somewhere, then you don't mean a fixed distance or even a time, but literally a fixed point.
I'll see you at fifteen meters
Time can be abstracted to a point when you talk about a specific clock time only, e.g. "I'll see you at 4:50pm."
You could only say "I'll see you in fifteen meters" if "fifteen meters" was an amount of time. You would need a lot of context to make that work, including establishing the "rate" and "time" parts of the distance = rate x time equation.
A: I can run 2 meters a second.
B: So can I.
A: Fine, I'll see you in 10 meters.
Even then, this sounds weird.