How to use your GPS watch intelligently in a marathon:
1. Set it to show average lap pace, with autolap set to 1 mile. If you can show a second field, have it show the elapsed time. Everything else is a distraction.
2. Sync your watch the night before to reduce satellite search times. Turn on your watch 3 minutes before gun time. Hit the start button as you cross the starting line.
3. WATCH YOUR AVERAGE LAP PACE. Are you 30 seconds faster than goal pace after a quarter mile? SLOW DOWN. Are you 20 seconds slower than goal pace after a quarter mile? It doesn't matter. Don't panic. Keep checking your pace every so often so you don't do anything stupid. N.B.: At the start of nearly any race, you will be surrounded by people doing stupid things for the first 1-5 miles.
4. From mile 1 or 2 on out, the watch is your pacer. Get on goal pace. Stay on goal pace. On race day, this should feel easy for the first 15-18 miles. If it doesn't feel easy after 8-10 miles, SLOW DOWN.
A pacer you can run with is more effective at helping people hit goal times, but a pacer won't slow down for you if you drop off pace, either. Then again, a GPS watch won't try to "bank time" in the first 5 miles.