Why do I walk so fast and run so slow?

Today's run (street): 2.5 miles

Yesterday morning I took a walk with a colleague to Central Park to scope out locations for our division Fun Run. She's a fast fitness walker, but I was able to keep up with little effort. I typically walk faster than anyone I know and I always have. It's easy for me and it probably has something to do with my long stride length. So, why then, am I such a mediocre runner?

Before I started running I used to think that I'd be fast, simply because I walk fast, but four years of history tell me that's not true. I sometimes wonder if there are things that I do when I walk that get lost in translation when I run. My stride length when running always seems short to me and I know my cadence is typically 8% less than the ideal of 180 steps per minute.

This morning I went out again on a cool morning run and moved along fine, but I barely cracked the 9:00 range for my pace. I know that if I'd put in more effort I could have improved by 30 seconds per mile, but not much more than that. It would be great if I could apply my walking speed to running, but I don't see an easy way to do that. And, no, I have no interest in race walking, whatsoever.