Have you considered triathlon?
Great suggestion.
I took up running after giving up football in my early thirties, and after a couple years was running sub 2:50 marathons. Then I tried triathlon and got so fit that I ran a whole load of PB's the year I turned 40. At 19, with a good fitness background you have lots of potential and the time to develop new skills - and it will take time.
Anybody with a swimming background has a head start in triathlon, because the swimming is the most technical discipline. Mixing up the training with running, cycling, swimming and gym work is actually less demanding on the body than concentrating on a single sport (at least until you get to elite level).
Join a club to get the benefit of coaching - and enjoy whatever sport you take up.