Deciding how to lay out the internal organs and skeleton has always seemed sorta tricky to me, especially when you combine things that are well understood like a human torso with a horse body (I can take liberties with totally "fantasy"/made-up species like my Ventrannan Samoyed Taurs, of course), but most of my discussions with friends interested in centaurs seems to lead to very similar conclusions. You'd need larger (or twice as many) lungs and hearts to circulate the blood and get enough oxygen, but you can usually get away with having all of the digestive and excretory tract organs in the lower portion of the creature.

As for that comment... I suppose it depends on just how many things occupy those 24 hours out of necessity vs how many are left free--I get the feeling my artistic output would suffer a bit if I were working full-time, but then again, I don't have many other hobbies to occupy my time so maybe not? I was working 40+ hours a week at my fast-food job for a little over a year and still found time to draw enough to have daily posts. Having inspiration and motivation helps a lot, too, nowadays I find myself "art blocked" more often because it either feels like I've already done a certain theme to death, drawn a certain character too often, or just don't feel interested in drawing the ideas I do have or finishing the sketches I already have. But, when I do have that motivation, I can get a lot done! Sometimes that means going to bed a bit later than I should, but if I'm into a project that just happens sometimes.