OP, I'm pretty fit. I run long distances, play sport and weight train. I've been riding a bike for a few months, having eschewed cycling for a decade. Figured it would be a breeze, what with my cardio fitness and my arse and quads being all strong and that. I put the premise that I live in a place that is quite literally a national park named for its hills to the back of mind.
A breeze? Jesus.
Trying to get up those hills I was sweating, out of breath, felt sick I could have just sat on the floor
First run out on my shiny new bike, I couldn't make it up the 400m 12% hill near my house; I stopped after about 10m. On subsequent rides, I started to track how far I could get up, first to That Pub, then to The Launderette, then The Red Shop, and so on.
The first day I made it all the way, I was overjoyed. I was in gear 1.3 or something and going so slow I was nearly falling off, but I didn't care.
Now I test myself on how hard a gear I can get up in on fresh legs. I'm on 3.6 so far, and yes, it makes me want to vomit. This morning, after a weights session, I was back down at 2.3.
So yes, it gets easier. And despite having a decent lower body shape before I started, the biking has added a kind of "length" to my thigh muscles that I'm increasingly pleased with.