The Nutrience Oakville Half Marathon, 10k, 5k and 2k in September is a great way to spend a day outside of Toronto. Just a half hour's drive from the city, or about the same amount of time if you take the GO train, you'll run along the Lakeshore and through lovely downtown Bronte.
There is a walk option for those who wish to participate but cannot run. There's also a student 10k relay run. Teams of two to four participants can enter the relay and will receive a participants package.
You can pick up a free shuttle bus from the Bronte GO station to the start of the race. The half marathon and 10k run/walks begin and end at Coronation Park. The course is flat, fast, and scenic.