Great scenery and exercise, who could want more?  A brisk winter walk will help you stay healthy and connected to the Arnold Arboretum at a time when the landscape is pared down to its beautiful bones.  The emphasis is on fitness, with a healthy dose of information on seasonal plants and points of interest.   Join an Arnold Arboretum docent for this free walk, starting at 1 pm on Sunday, January 24 and lasting approximately 45 minutes.  Meet at the Hunnewell Visitor Center.  No registration is necessary.  After the tour, warm up with a cup of tea or hot chocolate in the Visitor Center.  For more information or possible weather cancellations, please call 617-384-5209.  For directions, log on to www.arboretum.harvard.edu.