Late 70's to early eighties running became mass participation. There was a huge boom 1980 to 1985. In the 70's there were rare races with up to about 300 competitors (except for the National cross Country where there were always a thousand or so). There was a massive bias towards Marathon running in the early eighties when many thousands of people ran many Marathon races around the country. I don't think that will happen again.
I have never seen running as popular as it is now. It seems to be massive in every respect except at the very top of men's distance running. There are runners everywhere. I think it is a good thing even though it is inconvenient for those of us who can't be arsed to get a early entry in somewhere.
I am glad there are poorly supported races still preferably in places like South Wales and Scotland as these are some of my favorites. If !0,000 turned up for Pen Y Fan I doubt that one would break the record.