answer:Not possible, you can still enter but plan on coming in beyond dead last. lol All you can do is start walking several miles a day interspersed with short bursts of jogging/running, for a couple hundred yard spurts. 3 weeks is not, even remotely, enough time to get in marathon shape. More like a year. You are also at high risk for injury going from couch to track, shin splints, strains, sprains. Don’t forget to stretch and do some warm ups before you hit the road.