Take it slow! And you should be good. If you try and do it all at once you will have a sore fanny! Break it up to an all day adventure! Bring snacks and plenty of water.
And by the way congrates on the the fight with your cancer! I will add you to my good thoughts page in my head. I lost my sister in law to breast cancer. She was 34. She didn't pay attention to her body and it got her. It sounds like you are a fighter and that's awsome. My next bike ride I dedicate to you lady!