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In the future I will give greater consideration to triple portaging although I loathe   the idea. The initial feelings will probably change though as I breeze down the trails all light and lively. Plus I will only have to employ the tactic early on when supplies are full. So what if it adds an extra hour to my portages for the day and perhaps I don't get as far. The only appointment I need to keep is with Mr. Big right at dark.

I'd bet more injuries occur in the first few days of a trip than the last ones?
All the workouts you do, unless they involve actual paddling/portaging are never going to completely replicate actual conditions. First day, your asking you body to do a " workout" that it hasn't seen in awhile, sorta like expecting to keep up your first time doing your PX90 DVD.
You might of ended up hurting yourself down the road, but then you might of found that a few easy does it days would have allowed you to acclimate to a " new" workout? You/We will never know for sure.

It's one thing to muscle a load occasionally but when it becomes a habit, your asking for trouble and eventually you'll find it, sooner if you already have physical issues.

You can either lighten your load so you don't have to muscle it up every time, 3x portage until/if you feel comfortable/secure or plan trips that you don't have any hard portages.
That would limit your options significantly. From what we know of you, I'd say 3X portaging ( at least at first) would be a lot easier to swallow than be resigned to the easy road, well traveled ;)

Now you know why I recommended some of my "tricks" for putting on heavy packs. I have no physical issues and can still heft a load, but considering what's at stake and taking the long view, why would I if there's an easier option 8-)