I went on this holiday at the age of 65 and even managed the 'tough' stretch of the Great Wall. Provided you are reasonably fit you will have no problems. It is a fascinating holiday and will make a marvelous birthday present to yourself.
On a practical note, I took some Chinese money with me but, as all meals etc are included, I actually spent very little. There are plenty of ATM machines. We went in September and the weather varied between hot and dry to cool and damp. Silk products are to die for as are the beautiful scrolls, two of which adorn my walls.
You will have a great time.