You're thinking of the Raikouji Chizuru story. I liked it because it was a good preview of what Kenshin would be like, and I loved her grandfather!
It's interesting to think about what Kenshin did during those ten years. He probably tried to avoid human contact as much as possible. However, we can be certain he probably fought in battles when he saw people being terrorized. He probably worked as a body guard for merchant caravans sometimes so he could earn some money. Other times, he might work in exchange for room and board. He probably spent his winters in the southern part of the country and his summers closer to the north. It certainly makes for endless fanfic possibilities.
For starters, I would recommend the FF.net C2 archive,
Between Those Ten Years, which is all about Kenshin's wandering years.