Well! If you fancy canoeing, the have a bash and see if you like it. But, to me, there's nothing to stop you carrying on walking but maybe not as far. It depends a lot on the state of health, I suppose, rather than age. I also think that, like barewirewalker has written, it's your perception rather than activity change that might benefit you (though try whatever other activity you fancy but not so much as an alternative rather, more as an enrichment to your life)
A mate of mine is in his late 70's and has no bother still walking the Wainwright hills and there was a bloke who I met in Llangollen, that I got nattering to and he'd celebrated his birthday by climbing Snowdon.
So if you still enjoy walking and wildcamping and can still do it, then why not carry on but mix it up with the idea you've got about using your vehicle to kip in while you try out some canoe courses and holidays.
I like what BWW has said and that's the way I look at being out in the country. I went on the Ridgeway last year (only for a few miles) and fancy walking along it again this year but I'm not really that bothered how far I'm going to walk, it was just magical being on the same footpath that has been in use for thousands of years.
So I think its not so much "how far" but how good anything that we do makes us feel, when we go out into the countryside.