How should you train for a hilly and muddy trail run when you are in your 50's and in pretty good shape?

If by "in pretty good shape" you mean pretty good shape to run 7 miles, then you should be fine, just train for running hills the same size as the ones you will encounter and if you can run in the mud, that will help. If you mean pretty good cardiovascular shape, it will be more difficult as your leg muscles may not be used to the effort and that will put a strain on your cardiovascular system even if you are in pretty good shape. If you haven't been doing any cardiovascular conditioning then I would suggest first running 30 minutes a day, 4-6 times a week, walking as necessary. When you get to a point you don't have to walk, then one of the runs should be extended by 5 minutes a week until you are running an hour at least, and hour and a half would be better, so we are talking 6-12 weeks. After that you will be able to run 7 miles and you should look for conditions that mimic the course (hilly and muddy) to best prepare. Not necessarily every day, once a week is fine. Good luck!