As others have said: you have no base so a)concentrate on gradually building up mileage and b)keep your cadence up to avoid stress on your knees. Fit is important, but saddle height is critical for mitigating knee discomfort.

You'll build up cardio while at the same time strengthening muscles, tendons, ligaments, etc. Even at that mashing is overrated. The best way to do significant climbs, especially as you get older, is to have the right gears and keep the cadence up to something reasonable. What is "reasonable" will vary. My experience is that when I have to I can hold 60 for a long period, but it takes a toll on my muscles; definitely something to be avoided on a long climbing ride if at all possible.