Hi Alex H. I think what Gydle said about cardio is great. About complex carbohydrates: That's not a term that's used so much in the paleo world. I suppose that mainstream nutrition would describe it as any chain of three or more glucose-fructose-galactose-type molecules (anything that's C6-H12-O6). But that includes a whole lot of stuff, and doesn't really divide up the conceptual world of carbohydrate in a useful way. For one, it includes starch like potatoes, and we love potatoes, and potatoes will probably refuel you nice and good, so ...
Basically there are grains, which are to be avoided. (With the major exception of white rice, for many if not most of us.) Then there is fruit and there is starch. Basically starch is just a whole bunch of glucose molecules strung together. You'll find it in potatoes, you'll find it in sweet potatoes, you'll find it in squash, you'll find it in ... white rice. Fruit will have a mixture of glucose and fructose generally (both separately and combined into sucrose -- not that it matters much).
Fruit may not be a "complex carbohydrate" according to the definition above, but who cares. Both fruit and starch will probably be excellent sources of fuel for you as an athlete. As for refilling glycogen and how quickly that goes, I'm not an expert, but I'm guessing that once you cut grains out of your diet you'll probably be able to listen to what your body tells you. If you crave bananas with your meal after you work out you should eat them. If you take a liking to potatoes with your meat, you should eat them.
So if you mean by "complex carbohydrates" anything bigger than a disaccharide (anything bigger than sugar, basically), then yes, eating a paleo/ancestral/human diet can include complex carbohydrates, and in great quantities: starch! If you mean by "complex carbohydrates" the "heart-healthy" whole grains, then no, you won't be getting those. And if you're wondering if you'll have enough fuel for swimming, then yes, you will: both starch and fruit.
(Many people profit from eating low-carb, of course. I was one of them. But it sounds to me like you are very active and don't have glucose tolerance problems. If you do, then that's another story.)