I need to prepare an internal product release road-map for product being built via scrum methodology, and have some difficulty correlating sprints to the road-map.

The main problem is that as I don't have effort estimations for every story, because these prepared immediately before each sprint, so I don't know what will make into which sprint.

I'm fine with changing the road-map as the development goes on, but need it to give at least some indication when things planned to be released.

So what would be the best way to do this, other then guestimating the whole backlog?

Thanks for any idea.