singlespeeding?
(runs away)
…seriously though, swimming is great. if you're new to it your technique WILL be awfull, consequently you'll spend all your energy just going very slowly, so it's a good workout. but you'll see quick improvements as your technique/fitness improve.
it's cheap.
i find it as good as yoga for mentally relaxing – 30mins thinking of very little else other than co-ordinating your movement and breathing.
and then you could enter a triathlon, but only if you promise not to take it too seriously.