Measuring Stride distance wirelessly
Use one shoe (master) to emit a sonic pulse. Use the other shoe (slave) to receive it and immediately transmit it back to the first shoe. The time taken (as calculated by the first shoe) to send and then receive a pulse is related to the stride length.

Radio won't work because the biggest delay is the turnaround time from reception to retransmission and, given the slight variations in turnaround time (due to CPU not being 100% on the ball) will be longer than the transmission time by a thousand times!