Well, much of the Thames, from Kew through to central London and beyond, has pedestrian ways, with a few cutoffs. Don't know the central part well for running, but I know Kew via Chiswick to Barnes, Hammersmith and Putney, and with a few little bits where you can't get access to the river (eg Fulham Football ground), you can run along the towpath to your heart's content. I think the centre is similar and there are always signs directing you back to the riverside.