This is hosted in Kenwood House which is a free entry if you ever find yourself in London on work or whatever.

It is right on the edge of Hampstead Heath which is some fairly large (~800 acres) and fairly wild heathland and forest that is a good place for long walks or running (quite hilly in places, great views (legally protected!) over London as a result), quite close to the center of London (~10-15 mins away from Kings Cross on the tube). Do check it out if you want some outdoor space or a good place for a run when visiting London