Hi Foxy
Am I right in guessing that you are a lady? If so, this may be relevant. I have had rubbish runs for the past 2 months. I thought that I was tired from some long training that I did in the summer, but resting for a few days didnt seem to help. Even short half hour runs are an effort. Trying to run at any kind of speed makes my legs burn and I feel terrible. Eventually I had a word with the coach at my club and he suggested that I may be anaemic. As luck would have it, I went to give blood yesterday, after giving with no problem 3 months ago, and was told that I couldnt give as my iron was too low. The nurse said it was ok for 'normal life' but too low to give blood (it was 11.5 and had to be 12)
When I looked on the internet for information, there was some good sites and they said that even a slight lack of iron from 'your' normal level (which has in the past, from blood being taken at pregnancy, been 14) will make a difference to running performance. One point I found very interesting was that lack of iron causes lactic acid to build up quicker, which may have been the burning pains in my quads?
Perhaps you should have an iron test?