get the "time out" books for walks using public transport. excellent directions. you can get lost if you really try hard to.
http://www.amazon.co.uk/s/ref=nb_sb_ss_c_1_8?url=search-alias%3Dstripbooks&field-keywords=time+out&sprefix=time+out#/ref=nb_sb_ss_i_1_16?url=search-alias%3Dstripbooks&field-keywords=time+out+country+walks&sprefix=time+out+country&rh=n%3A266239%2Ck%3Atime+out+country+walks
google "southern downlander" to get to the cheap downlander ticket offered by southern railway - you'll never find it on their site as it's rather well hidden. £12.50 unlimited travel on southern from london to ashford in the east and southampton in the west and buses around brighton and eastbourne. an absolute steal.
the last leg of the vanguard way - berwick to seaford is a cracker. there is a very pleasant circular walk from lewes which you can make as long or as short as you like. lewes up to the golf course and across to glynde. south up on to the downs and then to southease. from southease follow the south downs way until harry's mount then head back to lewes. easy to see on a map.
i quite like from southease to exceat (or up to berwick) along the south downs way (bus from exceat to seaford/brighton/eastbourne) for a nice leg stretcher.
folkestone/dover/canterbury way now very quick to get to from st pancras on the high speed service fro just a couple of quid extra - dover to london 1hr 7mins instead of 2 hrs. the north downs way from wye to folkestone/dover way is very pleasant.
closer to london are walks up or down the western side of the railway line between leatherhead and horsham (which takes in leith hill - highest point on the south east...i think). the walks are rather more pleasant than the map would suggest and also hold the fascination of the changing seasons. between leatherhead and dorking is box hill and surrounding country - one of my favourite places - which is quick and easy to get to.
and of course epping forest which is a great place for wandering about in.
by the way. use 25k maps. 50k is useless.
and...despite yet another price increase the netwok travelcard will pay for itself in no time.