I'm on the side of mental toughness being at least half of the equation, being in reasonable shape aerobically/anaerobically the other half.

Body type as someone else mentioned has it's pluses/minuses depending on what your attempting to do.

I used to backpack with a guy who had a 38" inseam, even though he was only 6' tall. On a relatively flat grade or stepping over logs, he could out-walk me any day. He rarely led the route though, becasue he suffered on the uphills/downhills. He also was a very moody person, which translated into others never knowing who was going to show up that particular day. So to me, being mentally tough, not a whiner/complainer means a lot.

You also need to know your limits and not let your ego rule the day.
Go to any gym and look around, you have any number of people, usually male, who are sacrificing form, to keep up with the other guy(s).

I double portage, even triple if the conditions warrant. If my life depended on me risking personal injury to make it out or save someone else, you could load me like a mule and I'd go until I couldn't go anymore.

That said, unless it's an emergency, you don't need to impress anyone, you just need to carry your share of the load, and enjoy the trip.
Common sense should rule the day especially on any kind of wilderness trip. If 3x trips across a slippery/wet granite slope is quicker/safer than staggering under  huge 1x trip load, then that's what you should be doing, solo or in a group.

Solo, your only limited by your personal strengths and weakness's, physical/mental. In a group situation, it's the weakest link in the chain principal. The trip route/gear should be based on what the " average" abilities of the members are, not the above avergae one's, unless they/you are able and willing to do what the others can't

You can always increase the difficulty/range of your route if you find you've underestimated everyones abilities,  but sometimes it's hard to scale back and keep everyone happy if you overreach

Another vote here for taking it easy the first time around.  Not only your portage load but also your route.  You can always ramp it up the next time but if you try to do too much the first time there probably won't BE a next time.  The learning curve flattens out a great deal after the first trip.

M_P -

Just keep in mind that, just like downhill skiing, canoeing also involves technique.  The better your technique, the less effort it takes.

For example, I took my 30-something son and son-in-law on a canoe trip a couple of years back.  I made it a "baby" trip as far as effort was concerned.  I thought it might be too easy.  My son was running marathons and my son-in-law is very athletic and always in the gym.  At double their age, at times I felt I was dragging THEM.

Also, keep in mind that a fall canoe trip can be like a fall backpacking trip.  I usually tried to finish up any hiking in the Rockies before the 3rd week of September ... too much chance of getting into some serious snow fall at altitude.  This season, I did my first fall trip to Quetico.  It felt colder/wetter than my typical mid-May trip.  Yes, it was an anomaly, but then that's weather for you!

As for me ... I've got the PERFECT body shape.  Its just that the portages are too long and someone added extra gear to my pack!   

dd

You guys are making the learning curve for my September trip look better and better.
Thanks, Marten.

If you're healthy, you've got the right body.

At 6'0" 210 lbs, I'm way too big to ever be competitive as a runner. But I like running, it's good for my back, and unlike all the competitive runners I've known, I don't have knee problems, achilles problems, stress fractures, etc  etc. So I'm not complaining.

I am another who hikes and canoes. I think you will find the portages in Q really easy compared to backpacking, even though your load might be a bit heavier. No quad-killing 1000 m elevation gain switchbacks for example.

For me, it was always a struggle to get away from the every-ounce-counts backpacking mentality, and accept double portaging as no big deal. DD's weight limits are about right for me too (hiking and portaging), so don't feel intimidated by the voyageur numbers you read either !

None of the portages on the route you mentioned are killers. Check QJ's portage database for details: sometimes not all maps show the portages correctly. I think one of the portages south of Jean was shown way off on (?) the Fisher maps, and I lost a half-day once because of it.

All of your wilderness backcountry skills are still valid in canoe country. The only difference I've found is a lot of the regs use waterproof plastic barrels for food storage instead of daypacks. You can spend money on them (REI etc), or sometimes you can get something just as good for free from Greek food stores who import their olives in throwaway plastic barrels which work just as well.