Personally, if you've run races before, I think you would be fine signing up and completing Dopey. I don't think "If it's your first marathon, I wouldn't do it" makes any sense to me. If you find a good training program and train properly, you can do it (I used Hal Higdon's Dopey program and it worked great for me). It was my first marathon last year and I had no problems and I felt just as good afterwards as anyone who has run 20 marathons. I also reached my goal of completing the marathon in under 4 hours.
They key to Dopey (and Goofy for that matter) for most runners is to take it slow for the half. For Dopey, you can take it slow for the 5k and 10k as well. I didn't take it super slow for either of those races, but I didn't even come close to PRing. The half was the key for me. I did it with my wife who is a bit slower than me. I still think that I would have been fine if I would have done it on my own and I still would have taken it easy, but running it with her was one of the best experiences of the entire weekend for me (since this was her first half marathon).
The hardest part for me was the early wake up calls, managing my time in the park with my wife, MIL, and niece. I didn't want to ruin their vacation by always leaving the parks early, but I was able to manage my time in the parks by arriving a bit later than we normally would and leaving fairly early, but not necessarily eat dinner and get back to the room. If you can manage all that, find a good training plan that fits your schedule (and be prepared to run 400-500 miles during training), the go for it. If I can do it, anyone can.
