It got me started on exercising, though I bought a set of weights not long after.

His fitness quest did not last long. Now he just does keto and talks about it all the time.

That's exactly how I remember it!

receiving those mimeographed lesson pamphlets in the mail.

Dynamic Tension !

In the early 1970s, I wrote to them and the cost of the course was way too expensive. When I didn't respond, they sent a second offer that was half the cost. Since I was only 14 or so, it was still too much. Then, a month or two later, they offered it at a very low price so I paid for it. When the "course materials" arrived, it was a series of photocopied pages that demonstrated a succession of dynamic tension exercises that were similar to calisthenics and used opposing muscle groups for the exercises.

Although I started the program, I have no idea if it was useful since I got into various sports that included weight training and what would come to be called aerobics.

Today, I have a modest daily exercise program that my doctor approves of for a man in his 60s. I don't worry about bullies at the beach anymore!

Then, they kept sending me "special" offers. I think it finally got down to less than $10 (late 50s, early 1960s) and I bought it. Wasn't much use.

I remember seeing it in a magazine and I was not very old. But, I could read.
So, I think I was around 8 or 10.
Mid 60s.

Now we have the rest of the story!

That and Sea Monkeys.

Early/ mid '60's when I saw it.