How about a canoe or rowing boat on your local lake Howard? Not quite the same, but good exercise.
I've recently taken up Paddle Boarding, that's a great core workout and uses a completely different set of muscles from my normal gardening/walking/cycling. A lot harder than it looks though, I'm still at the 'quite often falls in when trying to stand up' stage - but at first it was 'couldn't stand up without falling in' so I'm making progress!
