As you have found out, the looks of a tire does not equate to the condition. They are not the same as automobile tires and due to sitting a lot, and maybe in the sun, deteriorate much faster. If they are the original on that unit, and most units are a year ahead, the tires could have been made back in 2011 or 2012, making them 4 to 3 years old. You should never drive over 55 with these (or is it 65?) and they should always be inflated to the sidewall PSI rating, no more, no less. As you drive on them, they will inflate but the compensation for this is built in. Have your comprehensive on your insurance pay for the damage and get all new tires, insuring that the code on the tire is indeed indicative that they are new. Lots to learn and as you can see, I've been there too.