I run three times a week. I time myself at race speed once a week and the other two runs are run without being timed. I heard that a runner should ease off on the non race pace runs however I sometimes find myself pushing on days when I don't time myself. Does it make a big difference to performance if a runner over exerts on the easier days. I average 15 miles a week and usually run my timed runs over a distance of 4.5 miles (around 8.05 min/mile).