I was struck by this, too. He is a senior citizen who has now been on the move constantly for a week, back and forth to Scotland and Northern Ireland and London, on his feet for hours at a time for long services and ceremonies and vigils on hard stone floors and uncomfortable chairs, plus the processions ... my back would have given out long ago, and I'm much younger than he is! And on top of that, grieving is exhausting, and they have all (her children particularly) had to do so much of it in public. I hope they will get a bit of a break now, between this procession and the funeral on Monday. I think he still has to visit Wales, though, hasn't he? Can William and Kate go instead, or does it have to be the king?
This is more superficial, but I also was struck looking at the photos from the procession how much Charles suddenly looks like his mother. I wouldn't have said he did before, but there was something about his face and the way his mouth was set that was just exactly like her.
I also cried watching the Duke of Kent leaving the service at Westminster Hall. He could barely walk, and yet he was there and looked so determined to pay his respects. I don't think Princess Alexandra was there at all, I didn't see her in any of the photos. Two of the queen's cousins who have served nearly as long as she did. Many of the Kents and Gloucesters were there, though.