Dulwich and it's surroundings is a great place for marathon training because there are so many parks and green spaces. You can do a loop of, say, nunhead cemetary - peckham rye - dulwich park - crystal palace park - belair park - southbank uni playing fields - finish at brockwell park. Once round each park and that's about 15 miles, which is a good solid marathon training run out and very hilly to boot.