I did this in 2016 at age 60. I am 5-8 and 160. I rented a Roubaix from West Maui Cycles; it had 50/34 and 11-32 and was like new. My biggest climbing training ride was 9 times a 3-mile hill repeat with steady 6 percent grade. By December of that year I had ridden over 6000 miles.
The ride was OK until the park entrance. After that, the accumulated fatigue, the elevation, the wind and the sun were very tough. I walked the last steep 100 yards or so.
I would recommend NOT riding back down. I was way too fatigued to make the hundreds of decisions required of a fast 35 mile descent. I rode down with my wife in the car and was glad I did.
Don’t let the modest grade fool you. This is a tough climb.