My bike commute is about 22 miles round trip, and it takes me about 90 minutes total on average. My route is pretty hilly with about 10 traffic lights along the way. It's dark in the mornings from fall through mid-spring. The biggest challenge is heat (and thunderstorms) in summer afternoons. Winter mornings were a piece of cake compared to riding home in 105-degree temps.
A 35-40 mile commute is very doable if: you're in shape; riding a road bike that fits; roads are mostly flat (as you said); you budget your time well; you don't feel the need to hammer all the time; your schedule is flexible if unforeseen difficulties arise.