Two years ago I decided I was going to start running between my house and the subway station each day on my commute, for a total of about five miles a week. Initially I was very interested in getting faster, and would time myself a couple days a week, trying to beat my previous best:
Unfortunately, after six months I was pushing myself too hard, and my knees started hurting. I stopped timing my runs and stopped running so hard, and they got better again. I'm still running, but at a gentler pace.
This experience was a good illustration of how optimizing for a metric can often bring you towards your overall goals for a while, but then continuing to optimize on it can start to bring you away from them again.