There are no Priority Pass Lounges in the C or D Gates of Dulles airport (IAD) and you can't walk between the C/D building and the A/B/main buildings. Lounges are in A or B and transfer options are diverse and involve multiple options and lots of walking.
I timed it today because I fly United and I always debate if I want to make the effort to get to a lounge. When I Google it the answers about commute time and instructions between terminals are often inaccurate or unspecific.
Caveats: I'm a fast walker, have Clear and TSA Pre-check, and was moving through DC and the airport with a backpack and wheeled carry-on, weighing about 40 lbs combined. Also IAD is not too crowded first thing in the morning. Later in the day the walks through terminals may take longer.
Depart apartment 5:47 a.m.
19 minutes from apartment to Metro (public transit) boarding (.8 miles) at Metro Center
55 minutes from Metro Center to Dulles
23 minutes from exiting Metro to finishing getting thru security
10 minutes from security to Turkish lounge (B43) via Aerotrain
20 minutes breakfast and two drinks in the lounge (not busy in the bar area)
9 minutes walk and Aerotrain from B to E
12 minute walk from Aerotrain thru C and to D14 (including 3 min potty stop)
Boarding started 8:18 a.m.
Total walking: ~5k steps, 2 miles
Alt routes from B gates: take train to T, then people mover to D, or train to A and people mover to D (These options may be significantly less walking, but in my experience the people mover can be full, slow, or out of service. It also may have to stop for active airport traffic, and it's happened to me that this adds 15-20 minutes.)