I wasn't happy with my performance in runs 1 and 2 of week 6, so I went back and repeated them. And while it was better second time around, they still weren't great (more of a struggle than I was happy with), so I really wasn't confident about completing the 25 minutes of run 3.
But, resisting my inner voice which thinks it still has the pace of an 18 year old, I listened instead to the rather more sage advice of everyone here. I set off slowly.
Slowly enough to feel the need to pretend, when I came across other people, that I was going this slowly only because I was just looking at the really interesting cormorants/egrets in the water, or what the fishermen were catching, or the surfers, or the nice view, or anything really... Vanity, vanity, all is vanity.
But then it felt OK. And after 20 minutes I wasn't the exhausted wreck that I had anticipated, but feeling good enough to smile and say 'good morning' to everyone I met. And, when I had completed the run and the 5 minute final walk, to jog for a few minutes back to my starting point.
So a good surprise really. And thanks to everyone for the advice.
I have told my grandson that he can call me Mr Plod in future.