My Garmin approach to London. Three data fields: current lap pace, total time, total distance. Autolap "on". Pace band on the other wrist.
The autolap function means I don't have to manually press the lap button 26 times, and I keep an eye on the lap pace for pacing. At each mile marker I check the time against my pace band. Whether the watch beeped 10 or 20 seconds earlier is irrelevant.
And don't moan when your Garmin says you ran 0.3 miles too far. Especially when it has you supposedly popping in for a cheeky Nando's at Canary Wharf.