Unfortunately there is no way you can go back once you click on "Mark Read". The good news is that from now on, all new messages that people post will appear dark red, making it very easy to find them. Once you've read all the posts you care to read, you can click "Mark Read" again and start all over. It's really a great feature.
The archives are unfortunately still broken, although you can search for specific recipes by using the search form, which works fine. The link below gives some information on what's going on with that.
I anticipate a resolution soon!
Link:
The Archives
If more of us valued food and cheer and song above hoarded gold, it would be a merrier world. -JRRT