I work from home I work in bursts of 2 hours. When I get stuck/bored (either because I don't have any more focus left or I'm stuck on a tricky problem) I will read, shower, go for a walk, or take a nap. When I come back to the problem, I generally find I am able to resume work.
I find that I get as much done if not more than my coworkers who must be in the office 8 hours a day. I also graduated from my PhD using similar tactics, while many of my classmates seem to LIVE at school.
My point is just that the 8+ hour work day may be possible for some types of tasks, but for real "thinking" work I don't think it is a good goal. You are burned out because you are trying to do achieve an unreasonable goal. Redefine your goals so you can succeed and be happy.
If your company has salespeople, they probably spend a large amount of time chatting. It is what it is. Some amount of it is healthy.