Originally Posted by FutureisNow
By pushing yourself do you mean in terms of distance or speed or both depending on the workout?.
Whichever you prefer, or can control better. Personally, I try to fix time, and try to improve how much I can get done in that time. That slightly increases volume, but at a controlled rate.
I agree on the variety - I find a day with 3 x 6 min intervals, 1 day of a 30 min steady, and 1 day of longer slow pace is a great mix for pushing, volume, and recovery.