I thought I'd actually share my routine since it seems to be working really well for increasing my swimming ability and flexibility which I think are super important. I think any cardio also helps with breathing.

I start off with some gentle stretching or I use a stretch machine at the gym that puts you through a little routine. Nothing too nuts but rolling your shoulders, looking left and right a few times, spinning your arms outstretched in big circles that get tiny and tinier, standing on your tip toes and going back down a few times.
Try some of these:



The thing about stretches it they wont do much if you hold them for more or less than 30 seconds.

After stretching I do 20 minutes on a cardio machine, I try to mix it up, then I move on to the weight machines


The seated leg press is pretty easy, and you can adjust the weight til youre comfortable but I used it when I was learning to walk and still could do it. It works your upper thighs but I also feel it improves your posture and resembles a similar motion you use while in a monofin

A good core machine can help a lot too. In addition to doing sit ups and different core stuff on the matts I use a machine that sorta looks like this


If you stick with just 1 exercise for your core you probably wont notice a diff but core is essential for monofin swimming ive found

I lift weights, and I use the different machines at the gym to do so as well as free weights too.