You can easily ignore all of the old posts by going to:
Settings (upper right menu next to "Log Out")
General Settings (on the left panel)
Thread Display Options (down the page in bold)
Default Thread Age Cut Off (Choose "Show threads from last year")
You won't see any posts older than one year in the past. (I believe you can still find them if you search for something.)
Now, get rid of ALL UNREAD posts from the past by going to:
Forum Actions (in the blue menu line above)
Mark Forums Read
You will be asked to confirm this action (read all the current "New" posts that you want to read before you do this.)
It doesn't delete anything. They just won't show up when you click "What's New?" above.