“extended trips”
“You just need to decide what you’ll be doing with the boat. If you’re just paddling go with the Q700. It is a lot of boat but, it’s light and narrow wih nice stability. If you want to camp and take extended trips go with the Q600 or 500.”
It looks like, if you can’t do “extended trips” in a Q700, you are carrying too much stuff!
And everybody
knows it bites if you poke it 
QCC Kayaks
Celia has it exactly right. It’s hard to give you good feeback without knowing your paddling goals. My suggestion is to paddle a variety of boats and put in some seat time.
And it all depends on where you what to go and what you want to do. While my wife Rebecca finds that her QCC400 works well for her, I prefer my QCC700. The 700 is a long narrow boat that takes a big motor to make it go fast - I’m still working on that part of the equation. The 500 is a freighter, a boat you could camp out of for a month.
Places on LI
There are some good places on LI where you could go to try out boats that might give you a better fix on fit and size before pulling the trigger. If you haven’t had instruction, there is still good weather to take a couple of basic lessons to understand what features in a kayak will serve your needs. Given what these boats cost, it’s a few bucks well spent to avoid buyers’ blues.
And while QCC does have a great return policy, it’d be a lot nicer to hold off purchasing until you have reduced the likdelihood that you’ll need to use it. I’m sure they prefer to have their boats act like college graduates - go out and find a good home and stay there.
Peconic Paddlers is one option, and there is an outfit in Riverhead that is a sattellite branch of a very large operation on the North Shore. All serious kayaks - these are not targeted at people who want Swifties, more what you are looking for.