I used it in a "scientific" way. If you were able to put the same person to do two exercises, one 60 minute easy run and second time 60 minutes tempo run (and p

I used it in a "scientific" way. If you were able to put the same person to do two exercises, one 60 minute easy run and second time 60 minutes tempo run (and possibly also time travel inbetween both sessions so they are in the same state at the start) you would measure that in the universe where you did the tempo run you would improve your fitness more (unless you were somehow injured/overtrained and not ready for the tempo run). This is where the part "per hour invested" comes. Most people can't do an hour of tempo run every day without getting themselves overtrained and injured. And if they could do it physically, it would be enormous mental challenge. Switch to most of us like you and me -- an easy run provides most of the benefit per hour invested but in a sustainable way, meaning you will be able to invest more hours and do it consistently. And you will be more likely to stick to do it every day if you find it refreshing and relaxing rather.