Hi there!
Me and my two friends are going to do the Blackpool marathon next year and it's our first one too. We did the Blackpool half marathon this year and we really enjoyed it. That is the furthest we have run so we will just be glad to finish it next year too!! We've decided to take it easy over the next few months and then start to pick it up in October. I think we have also discussed doing a half marathon later this year as a practice so I think the 15 mile one you have planned is a great idea, plus I think it gives you something to aim for as April seems quite far away at the mo.
We live in Blackpool and avoid the promenade when we are training as it can be a bit boring running in a straight line but it is a nice route on the day of the race. But seeing as though it will be 2 laps next year I will no doubt regret saying that!
My worst part of the half marathon was around the 7/8 mile mark where you run for what seems like ages on the lower walk of the prom, as there were not a lot of people watching and I think i just had a dip in energy.
I'm sure you will enjoy it though, we got chatting to people on the way round and everyone was really friendly. There were lots of serious runners but also lots who were doing it for charity, just for fun or for personal achievement.
By the way, it is pretty flat most of the way although there is a hill around the 10 mile mark where you come up from the lower walk onto the upper promenade but it's not steep :-)