Take a lunch break - to run! Running during the day provides multiple benefits: daylight, a break from the workday, and a chance to avoid eating a heavy lunch.
If you can take an hour for lunch, then focus on speed workouts - tempo runs, hills, speed sessions, etc. These efforts can improve your pace, stride, and running economy.
Don't worry about tiring yourself out for work when you get back. A mid-day run can reduce the muscle tension caused by sitting all day, brighten your mood, and leave you energized and ready to work once you return to your desk!
Keep these essential items handy to get yourself office-ready after your run: