I went through something similar a couple of years ago when I started exercising by walking. The pains stopped immediately I stopped walking, and only started when I reached a good brisk pace (or uphill). At the time the only board I was frequenting was the AOL diabetes board, which tends to be dominated by a couple of (sorry, but this is the only way I can think of to describe them) know-it-alls. They were absolutely certain that what I had was a serious complication - peripheral arteri... something! Some kind of thickening of the arteries which was preventing oxygen getting to the muscles.
Anyway, I googled it, and the symptoms sounded similar, so I went to the GP and told him my fears. He was extremely sceptical but agreed to send me to a consultant as I was worried.
The consultant was incredulous that my GP had referred me because of my age: apparently people don't get this condition under the age of 55 (I was 43 at the time). The consultant (a cardio-guy) checked me out and pronounced me fine.
After a while the pains just went away on their own, and I felt very silly! Have since lost a lot of weight and am now exercising every day with no pain. I think it was just my muscles complaining about propelling my 15 stone at a brisk walk!