Part of being a coach is trickeration. Adding in intervals that give the athlete something "fun" to focus on while in actuality they are getting in 40 minutes of base running which makes me happy. This workout is one such workout.
15' easing into things and getting the HR into z2
Main Set: 5 rounds
1' @z3 - think Half Marathon effort
4' easy running, letting that HR drop back into z2
z2 = zone 2 as z3 = zone 3.
(25' main set/40' total)
5' easy, slower than what you think you should be running.