The pain invariably occurred 36 hours after the event (DOMS) and I was often lulled into a false sense of recovery due to my legs feeling quite good after 12-24 hours.
The symptoms are due to microtrauma in the muscle tissues, minor tears causing inflammation and a build-up of tissue pressure which eventually reaches nerve endings, causing pain. This can last for 2 to 3 days before gradually diminishing. It is important not to attempt serious running until the muscles have recovered, otherwise further damage may be caused.
The best measures to help the problem are ice baths or ice packs, but DOMS takes its course. Should you be a fell runner and train on the hills, regular downhill running will inure the muscles to the activity.