Depends on how much couch potato we are talking. I find it very hard to work up an elevated heart rate while swimming since I only know breast stroke and am not

Depends on how much couch potato we are talking. I find it very hard to work up an elevated heart rate while swimming since I only know breast stroke and am not that good at it. Increasing your HR with swimming requires good technique which is a large barrier of entry. Even when I was at my least fit I would not have gotten that much from swimming or walking, presumably because I did small walking in my every day life like taking the stairs instead of the elevator. So obviously walking works wonder but only up to a point. I recommend walking and swimming for total couch potatoes but I personally I find them less useful as soon as you get a bit fit (unless you learn good swimming technique). Biking on the other hand makes it easy to pick your desired level of effort while keeping impact on joints relatively low. As a downside, unlike running or walking, it does not do much for your back health. Your spine and back muscles benefit a lot from walking/running. So biking and walking can be a good combination, or biking and running. And 100% agreed that your joints need to be trained gradually, your cardio and muscles improve faster than your joints. And while this is universal it is more important in running than e.g. biking.