I totally agree with everyone here!
My all-time favorite running route is from when I used to live at Waterloo:

From Waterloo over the walking bridge to Trafalgar Square, down the boulevard towards Buckingham Palace, then northwest towards hyde park corner cutting through green/st james parks, then doing a one-around of hyde park via marble arch/lancaster gate/ kensington palace gardens/ royal albert hall, back to hyde park corner.
Once around just Hyde Park is exactly 7.7km according to the google pedometer

Another fabulous running route is in regent's park in the fall or spring, lots of beautiful greenery!