literally all you need in terms of cardio is an old road bike and some hills

bike manufacturers have memed 40 y/o into buying lycra and fancy carbon bikes so they can larp like they are part of the peleton
and the pro cyclists are like the pro runners - all skinny for max efficiency, but that doesnt mean you shouldnt train to do a 20min 5k, biking is the same

reality is climbing out of the saddle is kino, and the most fum cardio