And for someone in between. I can run or bike solo every day and push myself to new personal records (though I never did break the 23 minute barrier on a 5k run). But strength training (primarily bodyweight) and yoga (picked up after a back injury left me crippled for a good chunk of a year), for me, require some other factor for me to stick with it for more than a month or so at a time (which is better than none, but doesn't help me get where I want to be).

Taking a class got me to push myself much further and harder than I ever did solo, and these days I'm making do with online classes because they keep me going in a similar way (though without the same social commitment since it's not actually social).