Walking as fast as running?

An interesting thing happened yesterday. I did my usual run around Lake Calhoun. Did it during the afternoon, and although there was a nice breeze, it was pretty warm out. Wanting to up the pace, I started out at a decent clip, and on the east side of the lake the breeze felt good. However, once around to the west side and cruising at a decent clip, I started feeling pretty cooked. Determined to press on, I slowed down, but at about 2/3s my body made me walk for a bit. Of course the danger once you walk once is that it's easier to give in and walk later. But as determined as I was not to walk again, the body shut me down two more times.

But here's the interesting part. In spite of walking three times, my time was almost identical to the time of the last run, when I didn't walk at all! WTF? One second per mile slower, in spite of walking three times. I'm not a genius, but it seems to me that I went out way too fast, and after walking pushed myself to make up for it each time, thereby cooking myself again and again. This is interesting. So how much was heat a factor and how much was starting out too fast? I'm going to run in the morning tomorrow, and that might answer some questions.