I had a look myself, not specific on there about Rochester but I then went onto Bing Maps, put in 'Rochester', switched on their OS map (its free) and saw that from Rochester Castle there is a walk (identified by red diamonds) south towards Aylesford following the right hand side of the River Medway. At Alyesford (visit the Friary and have a look at the railway station, used as the Hornby model station) then just make your way over to the North Downs Way, loads of footpaths to choose from (join it near Kits Coty) and return to Rochester.