In my, thus far, short running life I have focused a lot on listening to my body and not going crazy when exerting myself.
Today is my first day back on the road. About three weeks ago I came down with something and so I took a few days off. It was hitting about a week off when I decided to go for another run. I wasn't feeling the best, and it would have been much better if I had just stayed home. I didn't listen to my body and I got taken out for another two weeks!
Tonight was my first night back, and although my run was labelled as 'Very Hard' by Suunto, I was under-performing compared to my usual level. I was sweating like mad, but I had to get back on track. I guess I should also mention that it was the first time doing 8:1 intervals too! (yeah I know...that would have had something to do with it)
Anyway, I know this isn't a major setback, but it is very important to listen to your body when running. If it feels like you are going too fast/hard, you most likely are. ALSO if you have to take a week or two off due to illness or injury, it is MUCH better to do that then to kill yourself off and put yourself in a worse position.
So don't do what I did, listen to your body always! Running should be enjoyable! :D