You could go out along the canal past St Katherines dock. From Fenchurch Street head down towards the river (should hit London Bridge), turn left and head to Tower Bridge. Go under the bridge, through St Katherines Dock and then pick up the canal on the left towards Shadwell Basin - the run to the basin and back would be about 4 miles I think. If you want longer you can keep going, just follow signs for Thames Path and I think you can get all the way to docklands.