The first four miles of the Viking Way between Woodhall Spa and Horncastle is a straight (it's an abandoned railway line), beautifully manicured footpath through the woods. The first 20 minutes is wonderful echoing bird song and leaves rustling in the breeze. After that it becomes a little boring as you can only see forward and backwards.
Occasionally you come across the inevitable piece of public art. At least this is relevant and well made... and it breaks the boredom.