I've been battling this issue for a while now, but after Friday's race I decided it was time to actually do something about it. Basically, when walking/running, if I'm going any sort of speed I'll end up with cramping/pain in the lower part of the outside of my left leg (calf area, but on the outside). I've tried running without shoes (which makes my knee hurt) and being fitted for new shoes at a local running shop. I've added potassium/magnesium to my supplements (I can only eat so many bananas) and drinking lots of water (which makes me have to pee way too much). I've slacked off exercise, I've ramped up exercise. I use compression socks. Nothing seems to help. The weird thing about it, though, is that when it occurs, if I work through it the pain goes away and just becomes fatigue. For example, at the Peachtree it started around 1/2 mile in and was gone by mile 3. Saturday was just sore muscle until I went to walk the dogs (which is rarely a casual stroll) and it started hurting again. So, tylenol and some rest and the pain went away.
So, for my own peace of mind, I've set up an appointment with my doctor later in the month. Not sure what I'm going to do in the mean time, but I'll just keep monitoring it.