On August 2nd I'll be flying out of Buffalo, NY to Fresno, CA via D.C. and then LA. My layover in D.C. (Dulles) is only 40 minutes, and my entire trip is via United Airlines. Having looked at the IAD airport map, I know that United flights arrive in and depart from concourses C and D, so I won't need to transfer terminals any way other than by foot. My question is, providing the flights are on time, how long does it take to walk from C to D maximum? I'm nervous about missing my connection, and am trying to plan ahead as much as I possibly can.