"Yes, you really do need to try them on - too bad you can't rent shoes for the weekend - there's an idea!"

My local running store guy just switched me into Brooks Ghosts and said that I had 30 days to run the crap outta them and to bring them back if they weren't working. So although I'm not recommending that you abuse the policy, you may be able to exchange them after a week or two. So far I'm still as slow as ever, but I kinda don't think I can blame the shoes.