Scenario.
There is a 4:00 miler who runs 60 miles per week. 1 Interval workout at 4 minute pace. 1 Tempo workout at 5:00 pace. 1 Long run on feel. All easy runs are on feel but nothing faster than 6:30. This is the only training the runner does.
Now
The runner keeps the same volume of 60 miles per week and effort level on easy and long runs but beings doing the same interval workout at 4:30 mile pace and starts running the tempo at 5:40 pace.
How long could he remain in 4:00 shape? How long will it take for him to regress to a 4:10, 4:20, miler?