Honestly, only you can make this decision.
Personally - I knew I was going to run the WDW Marathon as my first marathon in 2016, but the minute I even thought about doing Dopey ... I knew I was going to do Dopey. It wasn't really a question, though at the time it seemed like it. I was running a 10K-10 Miler-Half Marathon a month at the point I registered anyway, so I was comfortable with the concept of the distances. If I was hesitant about the marathon distance, I would've done a marathon first before I did Dopey - just because it CAN and will be overwhelming ... there is so much on the line when you get to Sunday, and in my case, I had family and friends from out-of-town that had traveled down to Florida for the weekend to support me. I would have been devastated to not finish and disappoint them.
However, I'd done three simulation weekends leading up to Dopey (two back in Texas, one here in Florida) so I felt prepared. Unfortunately, the Florida simulation weekend fell over Christmas - with a 10-miler on Christmas Eve and 20 miles on Christmas Day. I'd never trained for anything like this before, so it definitely was a shock to my family - but luckily they were all-in with me from the moment I registered back in April.
So that's another thing that doesn't get mentioned enough, I think. To be successful with Dopey (and personally, with it being my first marathon), is you have to fully commit to training. That means running on vacation. Rearranging your personal life to get the miles in. Skipping out on nights out or other events because you have that long run the next morning, even though you JUST RAN TODAY. For me, it was easier than most because I wasn't working and training became my "full-time job", but it still sucked sometimes. My husband is really busy or gone from town for work for the entirety of May-June-July, so I wasn't inconveniencing him with my run schedule or a house that constantly smelled like a locker room or a geriatric center (yay, BioFreeze!).
Also, if you're just starting out on your running journey ... expect your costs to skyrocket exponentially once you start training for Dopey. You're going to need way more "stuff" than people who have been running for a while. More socks, more sports bras, shorts, run shirts, etc. - unless you like doing a lot of laundry (and what even IS laundry?!?), not to mention hydration equipment and the like to help you get through summer/fall training, extra fuel, a gym membership or access to a treadmill in the event you can't run outside.
@Ariel484's training journal en route to her first official Dopey really sealed the deal for me - I saw what she was doing, knew I was either already doing most of what she was doing OR could get it to that level and knew I'd be fine with my first Marathon being during Dopey. I suggest reading a few older training journals to get a feel for what the training was like and decide if you really want to commit to all of it ... not just for the Marathon, but for Dopey in general.
And in the end - respect the distance. That's the most important thing about the Marathon. Or,
@rteetz might also tell you not to eat at Ohana the night before too, as good advice.
