Anything is as good as walking/running, just because it’s different. Make sure you do something for your upper body too.
I would say that biking, walking/jogging or swimming are all good, no wonder they’re the triathlon.
Swimming is the best because it’s gentle on the joints and uses the whole body (plus knowing how to swim might save your life one day). But it’s more time consuming, since you have to go to the pool and back, shower before and after, dry your hair…take care of your swimsuit and towel.
Walking/jogging is great, especially if you’re doing it outside in a nice neighborhood or park…if the weather is nice.
An exercise bike could just be there, in your home, something to do while watching TV so it’s more convenient.
You should do 2 or 3 different aerobic activities. The more you do, the more different ways you can engage your body and the less boredom.
You can also do yoga, for flexibility and peace of mind, which is as good a goal as muscle mass gain and heart fitness.
You might want to use a heart monitor to get an idea on your cardio workout, whatever activity you are doing. I personally get the same heart beat rate if I walk/jog at 6mph, bike at 14mph or swim fast (but not racing speed).
