Harder than you think, but I've only had very limited experience in a kickboxing class. To be clear, it wasn't a kickboxing aerobic class, it was real kickboxing. Anyway, it took a lot more energy than I was expecting. Especially in the first round where it was all foreign to me. The things that really threw me off were the weight of the gloves (as padded, soft, and heavy as possible), the tunnel vision/awkwardness of the helmet (is it really a helmet?), and the mouth guard. I'd never used that type of guard, but it was super hard to breathe in. It wasn't something like I had in the ol' HS football days (er, junior high days). It was like breathing through a straw.
I didn't really settle down until the 3rd round. I was also up against a guy with 3 more years experience that was holding back. Until I landed a solid 4-hit combo on him. At which point, he proceeded to kick my ass.