I think you got it backwards -- you should mainly train in that HR range if you want to see aerobic improvement.

70% of what... LTHR or HRmax?

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People write the strangest things!

Fleck

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Steve Fleck @stevefleck | Blog

during base building i train exclusively under 80%, including 70% and 80%.

70% to 85% of HRmax...I know Jack Daniels mentions it in his book on running (have it at home now so I can't copy the exact text)

I think it depends upon the total number of hours you have to work out per week. If you have unlimited time, sure, do lots of lower intensity work with some bumps up into the 85 plus levels for interval training, and you'll be fantastic. If you are like MOST of the more normal people in the world, I'd say doing MOST of your training in this very range 70-85% of max HR is a good idea. You don't want to go much faster than your best 5 K pace (although you DO want to do some training at your faster than 5 K pace), and training at lower than 70% of your max HR just doesn't pay dividends if you don't have lots of time to spend there. Just my opinion...I know I'm LOTS faster when I train in this supposedly "no-man's land", but, I don't run more than 20 miles a week, nor bike much more than 100-150 miles per week. I'm embarrassed to say how little I swim...my weekly mileage on big weeks is less than many people's single workout mileage. Still, I've had the fastest swim split, fastest bike split and fastest run split in my AG in some races. So, don't tell me it doesn't work.

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Quid quid latine dictum sit altum videtur
(That which is said in Latin sounds profound)