Yeah I wouldn't put any unnecessary pressure on yourself by entering more than one at this stage. Part of the fun of your first marathon is saying "I'm never going through that again!" at the finish line, only to find yourself searching for another one on the internet a week or two after, and getting those twitchy fingers to press enter for your next race.
It's my opinion only, and many people do it differently, but I like to reward my 4 months of focused training with a couple of months after the race without any specific goal. This way I can have some down time and enjoy my runs/spend quality time with the family, whilst recovering for the next marathon schedule.
Been debating this for a few days, then left my entry form at work on Friday so had to wait all weekend to post it. Finally got it posted last night, a gamble I know, but you've gotta be in it to win it (or at least get a place, no chance of winning it! lol)