Z1/Z2 duration

Hi Scott
Firstly, thank you for putting together such a brilliant book. If only I’d have had it years ago as an amateur racing cyclist, it would have saved me doing 000’s of junk miles and I may have actually won something.

My question relates to the duration of Z1 / Z2 training. I understand it’s a case of the longer the better and I often do 2 hours plus at 55-75% Max HR when hiking and cycling. However, I recently noticed that on my daily dog walk I clock up 15 minutes in Z1.

Is there any benefit in spending such a short period in this zone or is it only the longer ones that make a difference?

Many thanks
Best wishes

Rich