Wejo,
I'm on this sight 5 days a week (on average). First off, thank you for always asking us our oppinion on any changes to the sight, it is much appreciated to think this sight is built for runners by runners who utilize our oppinions in developing the sight.
As to the Puma ads/new format. I work as an advertising manager for print as well as online. I do like the Puma ad. It is very catching how it spans the top of the page and flows down into the right tower. The colors are vibrant and are easily distinguishable from the content of the sight. The "Letsrun.com" logo is a bit small, and for a newcomer they will not see your brand (it should pop out at you - which it currently does not). I think it's great Puma is working with you (even if for the short term) and for a first run trial basis I'd say you are 80% right on. For the sake of the content in your sight I would shy away from allowing advertisers to manopolize three sides of your wall. The top and right hand would have been a better representation for both Letsrun and Puma. By not selling all three sides, you will be able to maintain your content (keep in mind your content is what brings customers - your readers - back to the sight, not your advertising) quality as well as providing a results oriented advertising medium for Puma.
Having the message board skinnier makes it more difficult to read the topics, however if you didn't sell the left hand side you would be able to maintain content quality. Anyways, so many people are throwing in their oppinion I'll end mine before it turns into a research paper.
Good job, I like it just remember to keep your focus primarily on your content and secondly on meeting your advertisers needs.