as far as I know, there’s no way to access animation curves runtime, so can not calculate future foot positions from those. [you can get them in start from a clip , but that’s not blended dynamic]
what I would (try to) do:
gather foot position data runtime, what is the furthest point the foot travels from the root position at a given speed
so after a couple run cycles would have the furthest positions (should be measured at a given normalized time which again would be distorted if speed changes after that)
probably the best approach would be furthest point / speed