45 miles, but there is a sauna...

If it's a good bag and has enough adjustable straps then it shouldn't be a problem, I can cycle indefinitely wearing my PAC, I feel weird when I ride without it in fact.

Exactly, I mean if you get yourself a decent bag like a pac ultimate, or an Archies Archie etc then you should barely even notice its there. I've done several 30-50 mile rides with my bag on with no probs at all, a couple of them I was even loaded up with quite a bit of stuff: Laptop, Charger, tools, change of clothes, 2l water, miscellaneous camera equipment lurking in there as well such as flash brackets etc.
As Will says, nothing that your average courier wouldn't do in a day, and with a good comfy bag that fits properly, you'll be golden :)
Clothes wise though Hippys suggestion is really good, for just over a year now i've stashed all my work clothes at work, and if it means you get in on time/early most employers are pretty accomodating.