Based on calorie burn, that is all that matters. More miles equals more calorie burn.
Walking a mile versus running a mile will burn about the same amount of calories. However, there are a few differences. Due to running/sprinting being more intense, you are burning carbs(sugar), where as walking will burn more fat. Hence the cardio and fat burn zones for your heart rate.
So like i told him, the 4.5 miles is going to get a greater caloric burn(and mostly fat), but the 2.5 mile HIIT is going to be a good switch to get the most out of his workouts(building up cardiovascular endurance as well as burning the most calories) and keep the body from hitting a plateau.
So my bottom line was correct. Congrats if you can make someone sprint 2.5 miles and make them more tired versus walking/jogging 4.5 miles. Its not brain surgery?
Did you just say that "4.5 miles is going to get a greater caloric burn, but 2.5 mile HIIT is going to...[be] burning the most calories"?
Pardon me if I seem scattered brained, but there is so much to approach here I don't even know how to come at it.
I was kind of hoping you'd use the good old "fat burning zone" we are all so familiar with from the face of the treadmill. It's bullshit. And no, more miles does not equal more calorie burn. That myth is based off the belief that calories are measured similarly to how work is measured. In a perfect world, maybe this would be true, but due to the inefficiency of your body as you increase the intensity of the workout, you get diminishing returns in performance meaning you must burn that many more calories to increase the intensity that much more. Running recruits more muscles than walking and uses far more oxygen.
According to several studies, running x distance consumes approx 40% more calories than walking x distance (Syracuse University, Medicine & Science in Sports & Exercise Journal. Dec. 2005).
You can hit your "magical" 65%-75% for 4.5 miles and get some decent, fat burning cardio. But you aren't going to burn as many calories as a well regimented HIIT program (I think you agreed with this earlier so I won't go further into it). Your immediate source of those calories does not matter, it's the calorie deficit at the end of the day/week/month. As unused blood borne glucose is later stored into fat or low blood glucose will be pulled from fat.
HIIT/sprinting further has an added bonus of anaerobically working your muscles. Building muscle mass increases resting calorie consumption for the rest of the day/week/month.
See how your bottom line barely touches the surface?