how much freeway driving?

So all this talk about long rides got me to thinking about taking the gz250 up to my in-laws cottage to open it for the season when I have a break this spring. It's about 300 miles from where I live and usually takes me about 3 1/2 to 4 hours by car (avg. speed 75). I google mapped taking only non-highway roads and it said that the trip would take over 7 hours.

Now I'm not very comfortable on this bike on freeways to begin with; some of that's my novice status and some is that I just don't think that's what this bike does, or was designed to do. I'd rather take back roads anyway by motorcycle, but to cut down a little on the time, maybe I could mix in some freeway driving. So here are my questions: How much freeway driving at one stretch is acceptable for this bike, both in terms of how much damage it might be doing, but also in terms of how much I can take? Does anybody else use this strategy, or should I just take all back roads and forget about the time?

Keith