My understanding from your post is that you went outside and ran 26.2 miles (I am not sure what else an unofficial race would be, given that you "measured" the course by using a GPS watch). I think you need to run an official race for this to count, as the poster above stated, how can it be verified? The good news is that you know you are capable of running the distance -- so doing it in a race shouldn't be a problem -- just make sure you do it wisely, as to not injure yourself. Also, I think pattern is important...
If a record gets briefed that "Candidate X has completed [3 marathons or 8 5k races or 4 10k races, etc.]" then it is obvious that it wasn't a one time deal and can show a performance trend. My point here is you don't necessarily need to run marathons (or multiple marathons), either. You can do combinations (numbers above are just random numbers I picked), but showing that you have a pattern is beneficial and can send the right message to the board (i.e. you take physical fitness seriously).