I think this was mentioned before but it is better to have filters and a ratings system than subcategories.
You would have the main random page which everyone loves. Then, for example, if I am in the mood to look for serious running advice, I click a checkbox (or multiple checkboxes), click refresh, and the threads I don't want to see drop away.
Now I have a page that looks the same as the main random page but without the garbage threads. Then if I want to I can sort by rating and, voila, the best training threads are at the top of the page.
This is better than search in my opinion, but you still need the search function (an improved one since the current one sucks).
Maybe another filter option would allow you to ignore posts by a specific handle. Then the 430miler haters could filter him out, or the 430miler lovers could filter down to just his posts.
Another thing I would like to see is an option that allows me to select the number of threads that show up per page. Typically I scan the first 3 pages to see what has been going on, but if I could select "200 threads per page" then I would never have to leave page one.
I also agree with the people who don't want the goofy avatars and other garbage that clutter up the page.
Also, if you implemented the filter system, the way to get the old threads categorized would be to bump them up to the front page and then have users start selecting relevance tags for them. You would flag or color code the thread and people would know it is an old topic but in need of categorization. You have a ton of viewers, so if just a small percentage of them open the thread to help categorize it, it would be pretty accurate.
This has a dual benefit. Revisiting old topics may bring up new discussion about them.
You wouldn't want too many old threads cluttering up the main page, so I would say bump up 3-5 threads per hour. Or have them separate at the top so people know they are not current topics. After a few months you would have a lot of stuff categorized and the filter system would continue to get better as people helped categorize old stuff.