Checking Your Pace at Mile 1 and 5K
The 5K mark is the halfway point of a 10K, which makes it your most useful checkpoint. When you reach it, compare the race clock to your elapsed-time target. An early gap in either direction is easier to correct over the remaining distance than one you notice with a mile to go.
Check the course markers, not the watch's laps. If your watch has logged extra distance, its lap alerts arrive before the official markers, so the paces it reports can read faster than what the course itself is measuring.
Using a 5K Time to Set a 10K Goal
The RRCA pace chart developed by Amby Burfoot states that a 5K is a better predictor of a 10K than a marathon, because predictions work best when the reference distance is close to the goal distance.
Each 10K goal-time page, such as the 45:00 page, lists the 5K time that lines up with that goal under Daniels and Gilbert's VDOT formulas, so you can compare it with a 5K you have run recently.