I was going to suggest LHR-YYT-YYZ as well but it comes with a huge downside: means flying on smaller planes.

I'm no doctor but I suspect you're actually better off flying one long flight on a 777 and walking up and down the aisle periodically rather than flowing two shorter flights and having nowhere to stretch.

On AC you can pay extra for a "preferred seat" and get the bulkhead seat on a 777 or 333 which has a fair amount of space to stand up and stretch in. (Or is it the exit aisle that has all the extra space. I would check out the seat maps to be sure). On BA you could get WTP but it's significantly more expensive.

Also, I've seen it recommended to drink tomato juice rather than soda or fruit juice as the extra salt helps. Again, I'm not a doctor.

As an aside, it's too bad AF isn't flying the 380 to YUL any more. That would have been your best bet to get space to walk around. When I flew it I even went up and down the staircase a bit for exercise.