Confusion and tedious detail about Forerunners

I was going to run a three mile steady session this morning, with one mile warm-up and cool-down. I set off, with the golf-course, Frog Firle, High and Over route in mind. The first mile warm up passed well enough, but then the Forerunner beeped and started to count down a four mile session, rather than the three mile one I was expecting. I decided that I must have mis-remembered the length of the session and prepared myself psychologically to run four miles, with a mile on the end, making a six mile total, and adjusted my planned route. However, when I reached the four mile point, the Forerunner sounded the end of the session. So I ran an extra half mile or so as a cool down.
That aside, and I was too slow too, it was a pretty early morning run.
Time: 1:01:47
Distance: 5.68
Pace: 10.52 (best 8.03, target 9.09, so well below target)