Time to Remeasure the Course

In my dissapointment at not reaching a 12 minute per mile average pace yesterday I forgot to note that it was a new PR for the daily walk. I edited that today. However, there was another dissapointment today--I tried again for a 12 minute per mile average and did even worse!

Mile 1 - 11:42
Mile 2 - 12:25
Mile 3 - 13:01
Average Pace - 12:22
Total Workout - 37:08

I started out even faster than yesterday and tried to maintain the same steady pace on the second mile but was way slower. I used the last mile as a cool down but sprinted the last 300 meters.

So why is it that I can walk 8 miles at a 12 min/mi pace but can't do it for 3 miles? The mile markers must be off. I searched the Internet and found several expensive measuring devices but then I came accross a couple of documents explaining how "official" road races are measured--they use a bicycle wheel. Sure, the official way is to use a wheel revolution counter, specifically a Jones-Oerth counter, but counting wheel rotations and calculating the distance should get a fairly accurate measurement. At least it will be better than the odometer on my car because I'll be measuring the sidewalk instead of the street.

One more thing--I've been trying to do strength training at least a couple of days per week. I did it today and on Wednesday when I walked my fastest mile--but was it really a mile?