I'm glad Ben found my tip useful. I hope MT gets something from it as well?
I know you have bad knees. Even if you don't bend them at all, pulling a heavy load that's partially off the ground is far easier than one that's sitting flat.
I would be willing to bet that more injuries occur lifting a heavy pack and sitting it back down than actually carrying it.
I always try to use one of my tips whenever possible. I'm also not adversed to leaning against a suitable object and taking the pack off, letting the object of opportunity take the majority of the weight.
All the exercise/stretching in the world can't reverse the aging process. At best we can slow it down. Sooner or later just bulling the load and not using technique will catch up to you.
I'm sure you like most of us want to be paddling well into your Golden years. Anything you can do to minimize wear and tear on your body we/you would be well advised to consider, even if it becomes a case of teaching a old dog new tricks.
With bad knees, your always have to wonder will they give out at the worst possible time? Sooner or later good knees or not we all take a tumble. Carrying a 80 lb load makes the odds of you maybe hurting your lower back even greater.
A simple muscle pull is painful enough but over the counter pain meds might get you thru

, slip a disc, your in real trouble especially solo.
IMHO you have a few options. You can lighten your load which you probably don't want to do ( I like my comforts too), or you can find a way to distribute the load a little more evenly.
Now to the real burning question

How does one take a sleek solo Kevlar canoe that weighs in the low 30's and turn it into a bloated 55# one

That's like turning Christie Brinkley into Kirstie Alley

They both might get you there, but which would you rather lug around