What could be causing me shin pain?

I am a more than novice runner. I'm in the military, so I have been running at least 10 miles per week for 11 years now. I recently became more serious and have decided to increase my distance. I ran a marathon 10 years ago, but in just getting back into, I'm running in a half marathon in 4 days. I started off training by running 5 miles and increased over a few months to 10 miles.

About 3 months ago, before I started training, I started having shin pain. I'm not sure if it is classified as Shin Splints though. The pain seems to be a muscle. It runs along the outside of my shins the length of the shins. It feels like that particular muscle goes into muscle failure. It isn't consisant either.

I thought it was shoes orginally. I bought 5 pairs of shoes in two months and finally found what I thought was the perfect shoes in the Asics Kayano X's. I had no pain for two months and it started again (this was during my training period. Didn't have pain in 8 and 10 mile runs.) When the pain started coming back, I bought another pair of the same shoes. I also slowed down on my running distance and frequency. It seemed to help some. I went for a 9 mile run the other day and around mile 5 the pain came back. I took four days off and tried a 4 mile run. After one mile I was literally on my knees in pain. The pain goes away after walking for a while.

I'm not sure if it is the shoes, my form or if I am just not taking enough time off to heal properly. Any ideas?

Thanks