no forerunner today!

I didn't use my garmin forerunner to run today. I noticed yesterday that in the park, where there's lots of trees, it's good at keeping distance and lap times, but it definitely isn't too good at keeping pace. I ran way too hard since the forerunner was giving me 6 minute/km times, while I was probably running faster, so it was like switching between 4.15 and 6 minutes/km. I felt it today in my legs that I kind of pushed myself yesterday. Today was the mid-week "long run". 4 miles on the program. I did around 7 km: 6 laps in the park and running to it and back.
Took my lap times on my Timex:
running to park: 1:08
lap 1: 5:44
lap 2: 5:41
lap 3: 5:36
lap 4: 5:40
lap 5: 5:27
lap 6: 5:20
running back: 1:29
total: 36:07
As one can see, the time are almost 45 seconds slower a lap. I just ran a little faster in lap 5 and 6, since it said to do so in the training guide, for the rest is was a nice quiet run. The lesson learned is that I'm only going to use the garmin forerunner when running on the street, and probably not display pace or speed, since it can be inacurate.