I have a Garmin Forerunner 935 on my wrist and a Garmin 010-12103-00 Bike Speed Sensor mounted on my front hub. I set the watch for one mile laps and start it running 20-30 feet from the start of the course. Every mile you can get audio or vibration alerts or both. You can also show lap distance on the screen. The speed sensor means that the watch is measuring distance based on wheel revolutions instead of GPS and—properly calibrated—this is far more accurate than GPS particularly in situations where you are doing a lot of stopping and starting.
I might not have bought such an expensive device just for measuring but I already had it for intentional exercise. You can probably find a much less expensive model that does the same thing. Their Edge 500 cycling computer will also do this for <$200 but it won't keep your coffee warm.