I mostly agree with the previous postings, but have a few thoughts to add:
1. Track meets, or at least the marquee events, were televised with some regularity on network television (no cable, really, until late 70s and early 80s and it was more a luxury than the norm back then). Think ABC's Wide World of Sports--two hours every Saturday and with a lot of Olympic-type sports coverage.
2. You cannot underestimate what regular doses of seeing Jim Ryun, then Frank Shorter and Steve Prefontaine on television did for distance running. Others, like Marty Liquori and Dave Wottle, helped, but the big three were Ryun, Prefontaine, and Shorter. Honestly, Shorter did far more to raise the profile of the sport than Prefontaine. Prefontaine's cult following largely came after his death. Ryun truly did get things going.
3. The numbers of truly competitive high school and college runners in the 70s and 80s compared to what we have now is probably exaggerated by the memories of those who were there. I don't recall seeing nearly as many people at the front of the pack in races as we see now at the high school and college levels. There was some exceptional talent in the 70s and 80s, but there really is more of it now.
4. Not only soccer, but also trendy sports such as lacrosse and hockey were not in the picture in the 70s. A typical high school in the 1970s offered football, basketball, wrestling, track and field, baseball, and cross country. Upscale high schools might have had tennis, golf, and swimming as well. Girls sports were just getting started in the late 70s. They basically had volleyball, basketball, and track. Girls cross country was met with disdain by many because it was perceived they couldn't run that far--or at least that it was not healthy for them to do so. So, the competitive running boom of the 70s and 80s was mostly a male phenomenon, Joan Benoit and Mary Decker notwithstanding.
5. While it is also true that most kids went out for sports in high school back in those days, this was largely a football phenomenon. Rare was the high school that had large numbers of distance runners. Football, frankly, always had more sex appeal. In all but a few schools, distance running was widely regarded as a fringe sport for kids too skinny to play football. Skinny was not what anyone wanted to be.
6. It is true that high school kids had a much broader base of basic strength and physical activity back in the day than they do now (though weight lifting was just then beginning to come into vogue). Kids didn't have as many outside jobs, but many more were kept serious busy at home or on the farm by their fathers. Pong, the first serious video game, emerged in the mid to late 70s. Physical activity hasn't been the same since. BUT, running footwear then was not nearly what it is now. Kids ran miles and miles in Converse All-Stars, and got all-star cases of shin splints. I hate to imagine how many cases of stress fractures went undiagnosed, but many of those kids just quit running because it hurt so much. Many runners also quit because of knee trouble. I remember frequently being yelled at by occupants of a passing car as I would run alongside streets, "You're going to ruin your knees doing that!"
All in all, a mixed bag. My assessment? The top talent stood out more then and top talent is always what people remember. There is more top talent now and the top talent is slightly better than then. There is much, much more talent in the average state cross country meet than there was 30 years ago.