Look in my picture above, bottom right.
So when you click at the top of the forum to read new posts, when you are satisfied you read all the topics you want you click the bottom link as in the picture above and all topics are marked read. The next time you come back it will only show you new topics since you clicked that link.
If you want to peruse more you can go back and click on the "Active Topics" link which will show you all active topics ignoring your mark all topics read command.