I tend to bite my nails, especially when I'm programming or deep in thought. Last year I decided to start walking whenever I reach that point where I am fidgeting and can't sit still. It's totally changed the way I work for the better. I walk between 10-20 miles per day, and can get a lot of work done on those walks. Highly recommended, especially for people living in places with good weather and beautiful scenery.
I can see walking 10-20 miles a day being detrimental to your health long term. Knees, feet, hips, somethings gonna give. Good cushioned footwear and insoles is a must to limit the damage at that range.
Your mileage may vary (apologies for the pun) but if you can, walk.