Honestly you don’t. On the other hand it doesn’t matter. People get off the couch and run/walk marathons all the time. People get off the couch and just walk them. It’s entirely up to you as to whether you want to try it or not. If you’ve been doing 25 mpw since Jan you have a decent base to tackle a training plan.
A half marathon at altitude on what is sure to be a hilly course? That sounds utterly awful. Sign me up.