Absolutely you can do that, presuming the climate in Walnut Creek is temperate. I commute year round (10 miles round trip) in office attire in Boston with very little trouble. On hot, muggy days (85 degrees plus) I sometimes change my shirt when I get to work, and I don't carry my wallet in my back pocket because it tends to wear a hole in my chinos quickly, but other than that, for short commutes like ours, wearing cycling clothes seems to be more trouble than it is worth.