I have read that the long run should be approximately 20% of your weekly mileage. I've been stuck around 18:10 all summer (6 races I think all 18:07-18:12) and for the past few weeks I've started doing a 2 hour long slow run (instead of 90 minutes) on 40 mpw which is roughly 38% of my weekly mileage! Well I just broke my plateau and ran around 30 seconds faster in the 5K. I didn't change anything else. Has anyone else seen improvement like this?