I only commute 7 miles each way. Once I 'bit the bullet' and bought a pair of spandex cycling shorts, I never looked back.
When I leave my lab (I park inside) I do put a pair of 'basketball shorts' on so as not to scare anyone. I also usualy just put on a cheap technical fibre T-shirt instead of a 'real' cycle jersey.
But really WHO CARES?? I stopped thinking about it. Do or do not, there is no care. NO ONE CARES WHAT YOU WEAR. I think that will be my new tag line!!!