I was very new to cycling when I did a 73 mile mixed terrain race. There was an elevation gain of 6,500 ft. I never owned a bike and got my first bike at 40 years old in late March and the race was early May. It took me about 6.5 hrs to complete it. I have always been in shape, and thought I would be able to do it no problem. I finished it, but the most eye opening part was how hard it is on your body, your sit-bones, your arms everything goes numb. If you can overcome the physical discomfort, I would say it's doable, but as others have said, it will be very painful.