All of us are aware of the importance of staying hydrated.
It is unavoidable that the aforementioned retirees will arrive. They will likely leave their water in the bus. By the time they reach the top of the pyramid the resultant difficulty breathing due to CHF will ensue. Make sure you have water to give to them. It would not hurt to brush up on your CPR skill as well.
Xunantunich was occupied from about 300-900 A.D. El Castillo is 130 feet tall, the tallest structure at Xunantunich, and the 2nd tallest Mayan structure in existence. The climb to the top is not too bad, but be very careful if you are afraid of heights, because once you get close to the top, there are no handrails, just very steep drop offs! But it is worth it to see the panoramic view over the jungle canopy of the Macal, Mopan and Belize River valleys, as well as a vast area of the Guatemalan Peten District.
I slipped on the wet stone more than once, even with my good hiking boots. Be careful, it's a long way down!