Climbing is a combination of both, particularly if the climb is lengthy. I'm from FL and I had no idea what climbing felt like until I moved to VA, and this past weekend I went out to the Skyline Drive/Shenandoah National Park area for a training camp. I'd never been on a real mountain before, and then 40 miles in to my first ride of the trip I was forced to chase my much lighter and faster teammates up a Category 1 climb (10 miles, 2,600 ft, average grade of around 5% with sustained stretches above 10%). Guess what? No matter how fit you are and how often you sit down or stand up, it's going to suck really hard. I'm lucky to have a compact and a wide cassette so my low gear is 34/30, which lets me spin ~90rpm at a very low speed (around 8mph). This means that once I blew up after trying to hang with the big kids, I was able to spin at a lower pace and catch my breath. One thing I found while on this trip is that it is really important to use a heart rate monitor so you know you're not going too hard too soon.