I know it can get crazy this holiday season trying to fit in your workouts, but Runner's World has a few great suggestions.
30-30-60: Run easy for five minutes to warm up, then accelerate for 30 seconds, reduce the pace for 30 seconds, then run fast again for 60 seconds. This last segment should be faster than the first acceleration, but not a sprint; gradually increase your pace until you're running about as fast as you'd want to in your next race. The middle segment is not a jog; simply ease up your pace to give yourself a short reprieve before the faster push. Do this 30-30-60 cycle four times, separating each with two minutes of walking or jogging. Finish with two 30-second accelerations (with 60 seconds rest) at the pace of the last fast segment. Add or subtract a 30-30-60 cycle for longer or shorter workouts.
Speed Loop: Find a traffic-free loop that takes you three to five minutes to run at an easy pace. Warm up, then run a loop, noting your time. On the second loop, try to beat your time by five to 10 seconds. Walk or jog for one minute, then go around again, trimming another five to 10 seconds. Do three to five more loops, taking another five to 10 seconds off each. Cool down.
Happy New Year!