I'll also add that Gopro 5/6 also has built in GPS path, speed, altitude, g-force, elevation gain and more... so you don't necessarily need Harry's Laptimer for overlay. The refresh data rate is higher than the iphone GPS as well so it's much more accurate. You just need to know your start/stop and you can do the overlay in Dashware (free).
Here is a lap at the ring with GoPro data - I didn't set up laptimer since this was a unplanned fun run in my fully luggage loaded rental car.
