When I went from not running after 18 months of having an injury that sidelined me to registering for a half marathon, I used one of the iPhone apps that is sort of like the couch to 5k programs. If you have an iPhone or android, search for one of them. They build up your endurance with a mixture of run and walk. It is not right for everyone, but for me, it worked to get me to stick to not pushing myself too soon too fast. I didn't want to re-injure my ankle and the program was doctor approved.