Thank you to everyone who gave me advice on pace yesterday. It all really helped . I worked out what my slow pace should be, using the pace calculator (based on 5k run time) and set a max speed alert on my Garmin.
It basically went off continually (!) but it did make me aware all the time and I discovered something interesting. I've never run slowly enough to talk before and so although I found it easy from a cardiovascular point of view, my legs were really finding it hard. They were achy and complaining all the way.
Anyway, I did manage to get into a rhythm (of basically constantly trying to find a rhythm!!), and just felt like I could go on and on. For the last few K I felt able to ignore the alarm and it was a relief.
The end result was that instead of my planned 7.5K, I did 9!!! It took me 1hr2mins, which is pretty good I think