[Martina]

Swimming builds you "soft" muscle, and I wonder if that is the reason your sources think it takes less time.

I like doing a stationary bike exercise about 1 hr/session and 3 times a week with multiple programs; hills(i.e increasing resistance) or endurance.

I also suggest doing weight training about twice a week when you are not doing your cardiovascular workout, since muscles are natural fat burner. Therefore after a few months of doing both, you'll have your regular cardiovascular workout and your muscles working for you while you are idle.