I'm 56, and yeah, strength does diminish with age regardless of what I'd like to believe.

For cardio and lower body strength, I run. Running is pretty stressful on the joints though (knees especially), so many people would be better off cycling for quad strength.

For upper body strength, I use free weights (because they're downstairs, and not somewhere I have to drive to). Anything is probably good, as long as all the muscle groups get exercised.

Although I don't have one, I have heard from many people that cross-country ski machines are the best single training device around. Usually not cheap though.

For mental toughness, I've never found canoeing terribly demanding. Long, ugly portages or monster headwinds maybe. Distance running or cycling in miserable weather is about the same as canoeing in miserable weather. Some days you need a lot of mental toughness just to make it through the front page of the newspaper without going berserk.

So do you really have to do anything before embarking on a longer canoe trip? Some people (not me) can stay relatively fit even without formal exercise programs, and would probably be able to complete a decent circuit with full enjoyment and minimum pain. But if you want to get close to your potential, and when paddling really enjoy getting into that optimum heart rate zone and staying there, I would guess most people probably need a couple of months training minimum. (Gotta have dem endorphins).

Time to go for a run.