Plan your run, run your plan

When I used to dive, this was the mantra: Plan your dive, dive your plan.  The point is, don't get down to 35 metres and decide to spend another 5 minutes just because you saw a big fish and then run out of air on the way back up.

In running there isn't the same risk factor, since you can always just stop if the worse comes to the worse.

Still, its good to have plan and stick to it.  When a race starts and everyone crowds across the line it can be very easy to go off a bit quick.  There's a considerable temptation to try and keep up with those people that keep streaming past.

The sole good reason for owning a Garmin is the ability to keep an eye on your pace.  That guy just in front of you is as open to temptation as you are.