The Los Altos 5K Fun Run has been taking place on New Year's Day for many years. It's the finale in a string of holiday events in downtown Los Altos. After the carriage rides, Festival of Lights parade, holiday shopping, and more this event is a welcome break. It's also a great way to kick off the new year with some exercise to work off all the food and drink from the last few weeks.
Registration isn't required and it's completely free. You can stop by the Los Altos Recreation Department website for more details. Basically, you just show up on Saturday morning at 9AM and you'll get to run with thousands of other people. Anyone can run, walk, or stroll the course (they usually do with kids in strollers, pets, etc) and you can stay in town for brunch afterwards. The race starts at State and Main Streets where the clock tower is in front of Citibank.
You'll also find updates on the Los Altos Online website with an article there along with comments.