I am considering comuting by bike to either Chiswick (8 miles-ish)or Park Royal (13 miles-ish) from Walton on Thames by bike. I have a choice of 'steeds' either my best SCR 2, Clipless, no rack or my hack mountain bike, with rack and panniers. What mods should I consider to the bike I choose? What is the collective wisdom of cycling with a backpack?, I always thought it was best to avoid if poss. I intend to aim for cycling twice a week initially. Any other advice gratefully received.