is bike fit / seat height proper ?
what gearing does your bike have ? chainrings and cassette ?
the average 5-5 @ 200lbs plus person will not be a gazelle up the hills - so good chance you will need lower gearing to compensate
spin and take your time - don't push too tall of a gear - especially on long climbs ... bad for the knees ... you will need your knees when you get older
I never had an official bike fit. that being said, when seated my leg is at a 10 percent bend and I'm pretty comfortable for the most part so I think I'm dialed in. No knee issues so far- either going 20 miles on the flats or even when I fell on the climbs.
I have a SRAM PG950 12-26, 9 speed cassette and I believe a Bontrager Select 52/42/30 crank.