All good stuff.
As you can see like anything else, we solo enthusiasts are all over the board on mileage traveled/ gear choices and comfort levels we want.
M_M_, along with KF are probably the hardest chargers among the solo paddlers here. Their both are in excellent shape and from what I can tell pick their routes and daily mileage goals as much for the physical challenge as anything else.
I'd say most of us are closer to what dd describes. Sort of in the middle of the road as far as daily mileage goals.
Personally I've had hard physically demanding jobs my entire adult life.
I've put in the 12-14 hour days, laying concrete etc. Can I still go hard and long, yes, but at this stage of my life, I don't see the need or have the desire too.
If I committed to a trip with others and the goal was hard and far, I wouldn't sign on, unless I knew I wouldn't hold anyone back.
Like I tell the kids at work, " You drive the stagecoach, I'll ride shotgun" ;) So far they haven't pushed me harder than I can handle.

The two biggest things at least to me are  being mentally tough, which I think is critical when traveling solo, and as dd alluded too, being safe while your out there.
I subscribe to the old carpenters rule, " Measure twice, cut once". THINK before you do something. Ask yourself, " IF the worst happens, what will I do? If you can't answer that, you should re-think your plan until you can.
The trip reports are full of humorous accounts of group trips where someone took a dunking/fell/injured themselves not critically but enough the others had to carry their load.
Many of these humorous group moments wouldn't be so funny in a solo trip :'(  A minor or even major mis-shape in a group, could be life threatening while solo.
Not trying to scare anyone contemplating a solo trip or solo paddlers who want to go for an extended length of time.
But if your a person who just takes things as they come, doesn't plan or think ahead, then the rewards of going solo might be overshadowed by finding your in over your head with nowhere to turn.
If you've ever been in a group and been temporarily " confused" about where you were, and felt that momentary twinge of panic at the thought of being lost, only to have the group experience bring you back to security, you can imagine the same feeling if you all alone and not prepared for it ;)