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One item that you can take outside and that will give you strength and cardio endurance is very hard to find.

 Everyone has a different take on this. Some feel they will get in shape as the trip progress, others want to be in shape at the start.

Ive exercised for along time, so i can't imagine not staying in decent shape all the time.  Tripping doesn't require "bulk", but muscle endurance/flexibility and balance. Having these will go a long way toward having a safe and enjoyable trip

Mental toughness comes from the will to persevere when you want too quit. I find most older people have more mental toughness than many of the current younger people do. You can be as young and strong as you want, but if your weak mentality and prone to whine, you'll never make your goal no matter what it is.

Do to the poor economic conditions in MI, I'm working a physically demanding job, with 20 somethings in a 12 hour day setting. I won't claim to be the best worker, but I'm close to it, even though at 61, I'm old enough to be their granddad ;)
 Life, like paddling is more a marathon than a sprint. The kids start strong, but fade fast and whine incessantly :(
 Focus on the task at hand, paddling/exercise and let the other thoughts go for the time being, Hard to do but you can if you work at it.  I'll take mental toughness/stamina over brute strength any day.

If you look at the Elite military units, SEALS/GREEN BERETS/RANGERS, besides being fit, they focus on the mental toughness part. Most of their drop-outs are from the lack of it.
 
It's true that the best exercise for anything, is doing whatever it is you like on a regular basis. Unfortunately not everyone can paddle everyday, so some of us try to find a reasonable alternative.

My good friend is a personal trainer who is always asked what is the best exercise for someone to do?
He says its the one you can do on a regular basis and not get bored with ;)
I'm a firm believer than something is better than nothing.