There are a number of viewing areas along the course. The start will let you see your runner or walker get off and then they come back at the end of the second mile to the starting area. This allows you see the participants at least twice.
At exit 232 Highway 40 and Empire there is a large area to park on the north side of the frontage road and this is an easy exit to get on and off so you can get to the finish.
Exit 233 Downieville Lawson is a good exit if you take the road back west and meet the runners before they enter the dirt path for open space or as they exit the path by the Animal Shelter.
Your next best place to see the runners would be along Colorado Boulevard in Idaho Springs before the finish, great place to cheer, just remember access will be limited and please to not make it harder for the runners to finish.
Bring your camera there are great shots along the way.