I need to do some more research on the approximate interpretation of various pulse rates as a metric for effort level, but the last look I had said 150 was closer to my aerobic level than the 140 I've been sticking to up to now for fear of damaging the engine. Certainly compared to other perceived efforts (mainly from exercise under coercion of various kinds when I was young) it felt like work, but nowhere near any lethal threshold, so I just stuck with it as much as possible. I hope that's a reasonable approach.
I've been way, way beyond what "150" feels like before, but it never felt like it was doing any good. And I'm guessing that up till now, my running effort has seldom bumped this level of exertion (regardless of what the pulse reading might be), yet that somewhat less intense effort has carried me along nicely. Put it this way: Never before at any time in my life have I been running along thinking about something else for a while before realising that I'm running along as I think of that something else. I suppose I must have been fitter for having being tortured for many hours a day for a very long time (and actually I could move to the lesser extreme of rugby practices, rugby matches, and the desperate struggle to keep up with the rest of the scrum for 80 minutes, or fartleks the teachers didn't dare risk calling fartleks in case of silliness breaking out.) I must surely have been fitter, but it never took me to a point where I was running along nice and relaxed, and just enjoying it. So this crude little "theory" is something like, "less is more", I suppose. Moving along in a gentle progression in the c25k way can take you further than pushing all the limits. Really. Because this way you can reach a point where you don't want to avoid exercise at all costs.
Oh, I'm forgetting to report on the spinning progress. I went spinning today. It was OK because there's this heart monitor you can play with, and I don't mind dripping sweat everywhere, it turns out. That should do quite adequately for a "spin report".