you coudl make a state machine that stores the current position, waits a certain amount of time...FINISH transition to the next state.
Next state you store current position#2, distance from last position, store distance as a float, after a certain amount of time, switch back to the first state....which would store the distance again.
This would just loop. over and over. You would then get a distance per time.
kind of a hack way of doing it...kinda weird how an object NEEDS a rigidbody to do the simple math.