If there is a Color Run 5K event in your area...GO! It's totally low key as far as "racing" - if you can walk 3.1 miles, you can do the Color Run. It was epic funness involving 10,000 people getting colorified. We had a total blast! Absolutely the best 5K I've ever done.
Before the race (I'm on the right):
And after the race (I'm in the center):
ETA: My weight in these pics is about 182.