Find the the routes you can handle without grinding out your knee cartilage. If you're taking on some 18% grade hills, that's very steep, and your cadence is 40, you need to learn how to stand up, for this cadence at very least, or you need to go to a 22-front triple, or you need challenge the hill, when it beats you, walk it the rest of the way, or to turn around, and work on going farther with more practice runs. Or you need to learn how to pedal really fast before you hit the steep section, so you can take it to the top, without ruining your knees.
Basically your quads send signals. You have to decide how much you want to strain them (and your patellar menisci, the former self-repairing and strenghtening, the latter not so much).
I was a test-guinea pig at UCSD School of Medicine. They set normal resistances, my speed was high ( I used to do 28 mi in an hour
, alone, no areobar), and instead of letting me ride for three hours at my preferred resistance, they cranked it up to make my HR go into failure range. I was happy at 300 W. I can do this for three hours. They weren't getting the fatigue results they wanted, becuse my HR was 150 and I could keep that up all day. They cranked up the resistance and made me see how long I could keep up 700 W. IOf course I couldn't keep that up. Five minutes, I was spent. I was in the anaerobic zone, and my muscles screamed, "Stop".
Thirty years later, I can't do the speed I used to. In the old days, nobody passed me. Now, a lot of people do. Even girls. It's okay. Age 29 vs. 59, speed 27 mph vs. 14 mph, weight 165 pounds vs. 235 pounds, it happens. I'm a millionaire now, versus barely making it paycheck to paycheck back then. Life happens.