Yeah, modify your drivetrain, whatever it takes. It's time. At 74, my lowest gear is 26 X 30, about 23 g.i.. That's actually too low, but it's good for a bailout. Best to have a gear you never expect to use! Did 114 miles, 8000' yesterday. Dropped into that low gear for maybe 5 minutes, but probably didn't need to. OTOH, max gradient was probably only 8%, nothing in the great scheme of things. We have an 11-40 cassette on our tandem. 4 years ago, my lowest gear was 26 X 27. When I was 60, it was 30 X 25. That's how it goes.
I did some large elevation gain rides before that, gradually increasing the gain. That won't be my hardest ride for the year. That's still to come. It's all about the gain, mileage not so much. Practice on climbs with similar duration and practice hydration/electrolytes and nutrition. What you put in your mouth is key. Only eat/drink stuff you've used in practice.
G.I calculator here:
Bicycle Bike Gear Ratio Speed and Cadence Calculator