I got into a really good habit of taking short afternoon walks when I worked in DC last summer and I'm definitely continuing to do that this summer. Taking a 10-20 minute walk in the afternoon really lightens my mood and makes me more productive. I often see people working through lunch and not leaving their desks all day, other than for meetings. I think that's really stressful and that taking a break actually does make you more productive. (There have been actual studies on it that I've read before.)
So, besides giving you great break from work, afternoon walks have plenty of other benefits! Doing something other than sitting in your office chair is great for your muscles and posture. Burning calories is great too and extra exercise is good for your health. Soaking up some sun will give you a good dose of vitamin D. Also, your eyes will have a rest from the computer screen. And finally, walking outside on a great day just feels amazing!
Bring an extra pair of comfy shoes to work tomorrow and take a lunchtime or afternoon walk...you'll be surprised at how good it makes you feel!