Accuracy
have you ever had the feeling your GPS watch is off? This happens almost on every run with me and I know a few spots were I know my watch will always show a wrong estimation of my pace, specially when running under cloudy conditions or when simply the satelite is in the wrong position. Doh
An example of my exercises on a track
400m interval sprints are my favorite and I aim for a 4:10min/km pace for each lap on the track. The easiest way to measure my pace on a track without a running watch is by simply having a look on my watch at half of the track and when I know my target pace is at 4:10 (100seconds), I can adjust my speed slightly.