Things to do

If you are based or wish to start from Harpenden, Redbourn or Hemel Hempstead, the Nickey Line gives you 7 miles of rural paths to run, cycle or walk the dog and children. It is great for beginner or nervous cyclists to avoid the roads although there are two crossings by Redbourn that require care.
At Harpenden you can join at the (large) end steps by the rail bridge on Holly Bush Lane, at the ramp entrance (easier) by Highfield Oval on Ambrose Lane or further on near Roundwood Park School on Park Hill. If you're feeling ambitious you can cycle all the way to Hemel Hempstead, but you can take some of the cut-off paths that will bring you into the back of Rothamstead Park.
At Hemel Hempstead you can start at a variety of points including Keens Field and Hunters Oak.
At Redbourn, the Nickey Line runs around the south of the village by the Bypass.