I never thought about training plans until a few years ago when I bought an indoor smart trainer and signed on to Zwift and started wearing a heart rate monitor and seeing power (at least indoors).

Outdoors, I still don't have a plan - I ride for fun, but I do watch my heart rate zones and try to push it up more than just what my comfort level would naturally select. Doing rides longer than two hours and bumping up the climbing gets me enough fitness benefit, since I'm not doing any competitive events.

But over the winter, I try to loosely follow the Time Crunched Cyclist training plan for experienced century riders - and my fitness level was way higher than after a normal winter just randomly doing spin classes or on "dumb" indoor trainers. I've now done that for 3 seasons, works for me - I am definitely faster on my usual solo or long group rides.