I'm 60, a bit overweight at 200#, and a slacker, (ie; eat whatever and whenever I want, don't train in any specific manner, don't use a HR monitor or any electronics), and I can do a century with climbing at any time, as long as I have been riding.

I last did 105 miles with about 4000 feet (not a lot of climbing) at Christmas and I felt pretty good at the end despite only riding on the weekends in the weeks prior.

My point is with enough saddle time you can complete a tough ride without overthinking it. Training will make it easier and/or faster. Just work up to it, 50 miles, then 75 or 80, etc.