Here are some links to 5K's and 10K's mainly in Southern California, however there are some for other States and Canada.
All you need to do is put your zip code or area to find one nearest you.
The Race Place <------
go here and you will find many, below are some from this web site
American Cancer Society Relay for Life
National MS Society Walk or Bike
California Volkssport Association - Misc Dates
Spring Walking Festivals, Marathons & 1/2 Marathons around USA & Canada
Race LA & City Race - March thru May 2008
Camp Pendleton Hard Corps - May 17, 08
Palos Verdes Marathon, 1/2 Marathon & 5K - May 17, 08