> I never got any of that pounding on the treadmill or struggling to lift 16kg dumb bells

I think that's what was meant by "Most people do absolutely nothing of value in the gym."

There's a little bit more to it than that. I retained training services for the first few years I did triathlon simply because I didn't know anything about it. (What are these heart-rate training zones? What strength exercises are useful to triathlon? What are base/build/peak/transition stages? What are the correct biomechanics for strength exercise? etc., etc.) In the end I ended up reading a lot of books and pieced enough together that I construct my own training plans and do my own strength training - correctly - now.

At my first triathlon it was quite apparent that 80% of the participants hadn't done any of this. These days I'll see someone "pounding on the treadmill [or pavement]" or flailing through the water or riding their bike with a seat that's way too low or doing basic strength exercises incorrectly and realize that - indeed - all that time spent exercising is mostly wasted:

a) Pounding on the treadmill/pavement - usually too low a stride rate and/or incorrect running mechanics will lead to joint problems / repetitive stress injury in short order; b) Flailing through the water - hello shoulder surgery! Form is 10x as important as strength in the water. I've seen guys with well-developed lats struggle at 100 yards and skinny kids zip right by me. c) Riding bike with seat too low will eventually lead to knee and possibly back problems. Get a bike fit. d) Strength exercise is useless unless you are correctly activating the right muscles in the correct order. Worst I've ever seen: GHD machine being used with hip flexion range of 45 to -15 degrees. Ouch.

It is possible to get value out of a gym membership but mechanics are really important.