I commute 5 miles each way to work. A couple of short 10 % grade hills, but otherwise mostly flat. I wear Jeans, t-shirt and light rain jacket. Jacket is not cycling specific, just there for some wind protection since can get a bit cool in evenings and might rain. Jeans work fine. T-shirt is a bit soaked so i keep a fresh one in the backpack. Have been wearing nothing but normal clothes (jeans/t-shirts) commuting under 5 miles for last 4.5 years. Always have backpack with me too. Never been a problem, you'll be fine.