We just started using the Michael Clay Thomas series LA curriculum this fall with DS8, but we LOVE it so far. It is specifically designed for GT kids and it is not taking us a lot of time. I had a former jr. high LA teacher tell me this GT 3rd grade curriculum looks like the content she used to teach to 7th and 8th graders. I'm glad we started the curriculum at the beginning.

My son also writes in a journal daily. He writes about everything - book reports, video games, trips we take, what he's learning in science, etc. We occasionally do dictation practice.

Another resource I've heard great things about that we may add in at some point is writing strands. The books are designed to be worked on independently.

http://www.writingstrands.com/home.asp

I'm sure you can do it! We are working at least 5th grade level in everything and are done in 2-3 hours for the whole day typically (although DS tries to drag it out some days).

5 hrs seems like a lot of time! Good luck!