Karen, I actually like the idea of keeping the easy running going so I think you're doing a smart thing. It helps maintain some fitness, I believe helps speed recovery, and allows you to transition back into full blown training more quickly.
I didn't mean to suggest no more 300s. I'd just be cautious with them and, if they would continue to cause hamstring problems, then seriously consider dropping them. Of course, even if you do drop them completely right now, I'd suspect the effect on your marathon would be minimal, if any.
If Sean and you wouldn't mind, I think several of us may be interested in his 10 rules of the marathon. (BTW: I also know Sean. He worked on recruiting me to EC when I was still considering an education major. He's a great guy.)