The Pinniped Route, by Kerry O’Brien:

Go from Iwo Jima down to Memorial Bridge.
North along Rock Creek Park Bike Path past Watergate, Georgetown.
At 4.2 miles out, go around the tunnel, not through it.
At 5 miles out, trail passes the Zoo entrance... water 50 ft inside, bathrooms also

Optional, but highly recommended!!—stop to see the pinnipeds…the seals and sea lions at the zoo. Follow signs for either “Seals and Seal lions” or “Valley Trail”. The zoo has four pinnipeds—Norman, Maureen (California Seal Lions), Gunnar and Selkie (Grey Seals).

Head back out on Rock Creek Park Bike Path, back to Iwo Jima for refreshments J

Total mileage (10 miles if you do not stop to see pinnipeds (and that would be sad!) 10.5-11 miles if you stop to see pinnipeds.