Hi Zach, long time lurker, first time poster!
I have run three marathons since being diagnosed this past May, making it seven in total. I can assure you it is a very challenging goal, but in time you will find it's a great motivator and you just might get hooked!
When I was diagnosed I had a full marathon literally within a matter of weeks, so I had to go through a lot of tough changes, but once I got used to living gluten free I have found my races to be faster every time. My first race post-diagnosis was horrible, I had gotten gluetened the day before and suffered a host of ailments on race day (aching joints, stomach pain, the whole nine yards), but the two after that were much, much better and I have even set a personal record for myself with the last one. A gluten free diet does NOT slow you down whatsoever!
I run 6 days a week, one being a weekend long run, and I am more accustomed to distance, so my weekday runs can range from 5 to 11 daily, depending on my training load. I have found that training with "real foods" seems to work just fine, like raisins, gluten free pretzels, packets of honey, and even candy like M&Ms. But on race day I find that the gels are easier to deal with, and Gu, Powergel and Carb-Boom all have worked just fine. Do your research and see what works for you. I assure you my system is VERY trigger sensitive, and I have never had issues with any of the gels.
Since you are just getting started, I would recommend using Hal Higdon's plans, I find them to be good for those just starting out. You probably only need to maybe max out at 45 miles a week and get in at least one 20 miler and 2 18-milers. An 18 week plan is usually good for someone doing their first. You do it simply to finish, and then subsequent races you can work on bettering your time, tweaking training plans, etc. I make my own schedules and they seem to work fine.
I will say that you must pack your own food on race day, because the finish line spread will have next to nothing for you. I always bring a Think Thin protein bar to eat right away for muscle recovery. It's actually pretty tough to deal with actually, seeing all the awesome carb-laden food that you cannot have, but trust me, I am SO much better off without it. I could not be happier to be well, I think I have suffered for a LONG time and I am so glad to find out what was causing years of issues.
I wish you the best of luck with your efforts, and please feel free to ask questions!