So, further experimentation.
Encouraged by the progress of running 6-8 miles within the 140 pulse limit and feeling just fine, I decided to test out the idea of 'bimbling' along in one of my favourite races, the Black Mountains race. 17 miles and 5,200 feet of pretty ridge ridge running and vertiginous grass cliffs. I used to get round in under 3 hours 'back in the day'.
It was glorious day, clear skies and cooling breeze and all went well for the first 8 or 9 miles. Trailing along near the back of the field but comfortable. But then the wheels came off. There was no lack of strength or determination but my physiology just let go. I could not make any meaningful progress without my pulse shooting above 140 and I felt completely shot. Even a break for a snack and a drink did not improve matters so I had to pull out at 11 miles.

So, today, I feel fine. Normally my legs would be trashed after that kind of event but, because I cannot run to the point of muscular exhaustion any more, they are OK today.

My new self imposed limit for any mountain running escapades is now 10 miles!

Anthony, I hope Mrs Kay is doing OK?