Run 'em both. The first will act as a good long training run for the second and you might learn valuable lessons that you can put into practice relatively soon after. Get 2 marathons out of one "batch" of training! Don't put yourself under time pressure for either - you'll get enough satisfaction from being able to say that you did the two of them consecutively. And best of luck!
I've just entered...it was the pretty picture of the towpath on the website's front page that swung it for me!
I don't tend to expect too much of myself in end-of-year races so, coupled with not knowing quite what's in store, I won't be aiming for a great time ...they're more for the sake of keeping motivated when it would otherwise seem to cold and dark to get out and train/race!
I was the last to finish in my longest ever (past and future!!) race - 55 miles, last month. About 38 started, and 30 finished. I was temporarily fed up at the thought of not having my time recorded but was bloody well determined to reach the finish anyway. I did so, got my t-shirt and medal, AND was recorded in the results as the last finisher!
The vast majority of us in most events are racing only ourselves, in the sense that we're not out to win the race (or come 2nd or 3rd), so in my book there's absolutely no shame whatsoever in coming near the back or last. Someone's got to! And who knows, you might just inspire someone else to get involved, if they see runners of all abilities actually taking part!