Ok, it looks like the strenuous days are:
Day 3 - Dover Castle day. You mostly tour the castle on your own so you can control how strenuous this day is somewhat. The toughest part would be walking from the coach park up to the entrance. Be sure to save energy to walk thru the tunnels where they planned the evac from Dunkirk.
4. This day has been modified (for the good I’d say) since I did it so no comment.
9. Tintagel is the tough part. I’m a height chicken so I did not walk out there. Lanhydrock is somewhat flat.
11. St Michael’s Mount was the most strenuous site to me. I thought the path to the top was tough with it’s cobbled surface. Wear good grippy shoes for this.
12. This day has been modified as well (also a good thing!). Not sure why it would be strenuous as basically it’s a long bus trip back to Bath from Penzance, then a walking tour. Bath is a bit hilly but nothing like St Michael’s Mount.