I rode Haleakala when I was 52-1/2, 2-1/2 years after I had started riding. Had lost 30 pounds and gotten decent at climbing. Rented a Trek with a triple crank, "just in case." The round trip still took me over 5 hours. Reaching the summit choked me up, realizing how much healthier and fitter I was, than when I was 50. Would love to go back, some day. (The West Maui loop is also an amazing route.)
Good Luck, stick with a plan, and please keep us updated.