My best ever season I did the first half of the season of distance - all the base work, and then the season's training to time, sessions included as I didn't have a track or any way of measuring distance. I'm a big believer that should do some work, including quality work, off time as well as distance - if you always run a set distance, you work of preconceived ideas about pacing and timing. If you just run until the watch bleeps you may well find when you go back to the distance you have been running it quicker.