Just jumping in the pool to swim 20 minutes 2 or 3 times a week dropped my resting heart rate significantly week over week, as measured by my watch, although it's a low amount of exercise... I wake up to notifications for sub 40 BPM whereas I did not before.
When you start working out a lot of your strength comes not from your muscles getting stronger but your body learning to use them properly. I.e. your nervous system.
Are you doing interval training in your swimming? I think how you train can have a big impact on your VO2MAX improvement vs. time invested. That said long distance running does require a lot of time investment. I'm not sure if you can get to run a marathon by running an hour a week but I expect that answer is no.
What had the biggest impact on RHR was when I was just swimming 1km per session at normal pace (it took me about 25 mins, I'm a beginner). RHR dropped week over week down to 42. When I got a coach he had me do mostly interval training and interestingly my RHR crept higher although I did improve in terms of speed. Past month I've been lazier and training again to swim continuously and I see my RHR decreasing again. Unsure about the science of it and whether it applies to others. I don't think RHR is necessarily a marker for fitness but I thought it was interesting that pretty significant change was happening although I was maybe swimming 30 mins a couple of times a week.
It can be an indicator of short-term stress if it is higher than normal, for example. But the interesting part is the trend over time. As you get more fit, the average drops