I have no issues cardio-wise but I notice that it takes a few times in the spring with a canoe on my head to refine the mental aspect of my portage game. 

I run all winter and into spring, about 50km a week, right up until a few days before my first camping trip of the year.

Then I fatten up over the summer and fall, until it's time to repeat the cycle again!
 

I lack the personal discipline required for regular exercise, but for the past few years I have done a couple of things in preparation for tripping. 
1) I walk around the neighborhood with a canoe on my head.  There's a pond nearby, so I carry it there, paddle around, then carry it back.  The carry is around 1KM each way.  Added benefit, my neighbors think I'm nuts. 
2) I try to get on the water as often as I can. 
3) this one is a little different.  My ankles are not great, and I'm sure a creamed ankle is a very common injury in the park.  So, this year in particular, I've been doing these rubber-band exercises to strengthen the muscles in my lower legs/around the ankle.  I can feel the difference in the few weeks I've been doing it.  Much more solid on my feet, much less pain in general in the joint between foot and lower leg.  I had physical therapy for this many years ago, and it is (some of) those exercises I am repeating.

As suggested, biking and carrying a canoe around are two good activities that will get you into portage shape. But if you find it difficult to find the time to do both you can try this.