The hardest part was realizing that at 50 or so I didn't have the stamina of the younger students; I was usually the oldest person in the class. The warm-ups at the beginning of class were sometimes quite taxing and left me drained for the rest of the session. I learned to pace myself on the warm-ups so that I'd have the energy to complete the rest of the class successfully.
I'm enjoying the class a lot. It can get quite rough - right now I'm nursing a bruise on my ribcage, as well as a less painful but quite spectacular one on my forearm. You train with pads, which are supposed to absorb most of the force of your opponent's blows, but you're still going to feel an impact. (Especially if you're practicing knee strikes.) I like that, though, I like the realism and the stimulation from the striking and kicking.
Krav isn't a "martial art" in the traditional sense. It's basic hand-to-hand combat; it's street fighting. Good form is defined as whatever works. It's practical - I like that.