13.1 and 26.2 correspond to the distances (in miles) a runner should cover in a marathon.
The 26.2 miles is the official distance for a full marathon while 13.1 is just half of that road running event (half-marathon).
13.1 and 26.2 stickers are popular and are commonly used as car decals and temporary tattoos representing completion of a half or full marathon.
The Athens Marathon is regarded as the original marathon course. Legend has it that the first man (Phidippides), honored by a 24.85 mile (40,000 meters) run from Marathon Bridge to Olympic stadium in Athens died and collapsed after covering the distance.
The distance was changed to 26.2 miles at the 1908 Olympic Games in London to cover the ground from Windsor Castle to White City Stadium. 2.2 miles were added on so the race could finish in front of the royal family's viewing box. This added two miles to the course, and is the origin of the Marathon tradition of shouting "God save the Queen!" as mile post 24 is passed.
If you're a relatively new runner, you may want to consider the half marathon first. Read 13.1 reasons why this distance might be right for you to tackle before going after the marathon at Active.com.
Discover 7 things you're probably not paying attention to when you run (but should be) at The Huffington Post