I would add to the above answers by mentioning the Greensand Way. the section from Dorking, Surrey via Leith Hill, Holmbury St Mary, the Hurtwood, Shamley Green to Witley is exceedingly beautiful. Its about 30 miles but of course you don't have to do it all at once
I'd also recommend the Chiltern Hills in Buckinghamshire and Oxfordshire. You could walk along the section of the 'Ridgeway National Trail' from Wendover to Princes Risborough via Coombe Hill
and Chequers. This is about an 8 mile walk and both ends of the walk are on the Chiltern Railways Line from London Marylebone
You can also buy the 'Time Out Book of Country Walks' from bigger bookshops, which describes 20 plus walks in the south of England complete with sketch maps travel directions etc, although I would always suggest carrying the Ordnance Survey Landranger map for the area you are walking
in. These large scale maps show each area in great detail including all public footpaths and national trails. You can buy them at Waterstones or from www.ordnancesurvey.co.uk
www.surreyhills.org
www.chilternsaonb.org.uk
The South Downs are mentioned above; a beautiful area. If you just want to walk for a couple of hours go to the Kingley Vale National Nature Reserve north of Chichester and walk the circular trail there. Kingley Vale is the oldest Yew tree forest in Europe with some trees being several thousand years old. It is also partly elevated with gorgeous views. You can park in the villages of West Stoke, Walderton or Stoughton and walk up to Kingley Vale along footpaths, or take Stagecoach bus 54
from Chichester Bus Station to Walderton or the 60 to Binderton House and walk from there
Chichester Bus Station is opposite Chichester Railway Station on a line from London Victoria
Ordnance Survey Landranger Map Sheet 197 'Chichester and the Downs'
www.nationalrail.co.uk
www.traveline.info