I wanted to run my favorite trail today after my haircut. It was about 30 degrees, sunny, with a little wind. Normally, that would ensure a great run. However, to keep my heart rate below 125, all I could do was walk. I never generated enough heat to keep warm so rather than sabotage my MAF training, I gave it up and came home. Slowly, slowly, was able to jog six miles in the house. Yep, an hour and a half of that. And I even had new shoes to break in. Brooks Ghost 7.
I've written Dr. Phil Maffetone, the originator of the MAF training philosophy and asked if at my age would it be appropriate to let my heart beat get to 130-135? I'll be surprised if he answers. Looks like the questions and answers on his blog are several years old.