How to use a GPS watch to measure how slow you are

I ran a total of 4 miles in Spring Canyon this morning.  After about 3 miles, I ran almost as fast as I could for 60 seconds and then walked until I was rested.  I did this five times.  I was running "fast" but not wildly so - almost as fast as I could run for one minute, but not sprinting.  In my head, I felt like I was running 5 minute pace or a little faster.  Imagine my shock, shock I say, when my Garmin Forerunner 310XT watch said I averaged 6:37 pace.  That, my friends, is how you document the ravages of ageing.