Mixing things up during the long run can be helpful and useful to keeping the mind sharp and using different stimulus to help the body achieve a particular level of fitness that you might be chasing.
Majority of the time I'll tell you to go out and run for 75' (or however long) in an ABB workout (Aerobic Base Building) where you keep things in zone 2 with a little zone 3 being ok if on a climb. Today is an alternative to where the majority of the workout is still in zone 2 but we toss in a few zone 3 segments to mix it up and continue to build fitness.
AE 2's means Aerobic Endurance with 2min intervals every 10min.
20' easing into things, nothing higher than mid zone 2.
Main Set - repeat as needed till you reach the end of your workout.
2' zone 3 - don't force the HR up, just settle into a steady zone 3 feel and over time the HR will meet the effort.
8' back into low/mid zone 2. Walk for 30" or so if you need help getting the HR down, otherwise an easy shuffle for 1' should do it, then it's back to normal zone 2 running.
Easy jogging for the last 5min or so.
It's not much, but it'll help keep the athlete focused a bit more and they'll enjoy mixing it up a tad without zapping too much energy.