I'm sorry you found yourself in trouble. In the description of the tour it is well specified that a good physical shape is required and that it is not recommended for those who do not have it.
For your health, since you are not very young, I advise you to carefully read the description of a tour before booking it!
Even that it is not a private tour it is very clear, especially from the price you paid, since in Italy, but I believe in all European countries, a private tour costs at least € 60 per hour.
Having said all this, you cannot take the refund of the few Euros you paid (for the booking! Not for the tour!) for not having properly evaluated what you were booking.
I'm sorry but the only person you need to be angry with, is yourself!