This is really interesting, but I have to doubt it.

Before Christmas I had a month or so layoff, and having nothing planned this year I did a lab test in January to find my aerobic and anaerobic heart rates.

Since Feb I have only been running within my aerobic heart rate limit, building up from 30km per week to 70km per week now. I live in a flat area, so this was easy to do.

I have seen barely ANY speed gain for the same heart rate. I was running within 5s per km in Feb at 145-155 bpm that I am now.

I am slightly lighter, 2kg, so that doesn't make much difference.

I just don't believe in these one size fits all assumptions.

Five months saw almost no improvement for me.

What has happened is that I have no tolerance for any speed. I can run all day at 5:00/ km, but 4:45 is a killer.

Considering last fall I ran a marathon at 4:35/km, I have actually regressed massively.

My plan is in August to go back to normal mix of speed and tempo and slow for an April marathon.