Congratulations on gaining a place at BRNC. All I would say is (bearing in mind a self confessed mediocre AIB score) - don't look a gift horse in the mouth. Take the January entry, accept that you will miss out on your travelling (plenty of time to do that whilst getting paid to do it later!!) and that it will be cold(er) during the EMA's and PLX (or whatever they are called now). I went through on a half term flight that started in February, yes it was cold early in the mornings but a bit of exercise soon warmed you up.
Gets you out the other side quicker......
The advantage of this is that you show a committment to the Service whilst allowing you to plan to go to the RN Ski Championships (held every year in Jan) in January 2007 whilst you are on hold for the next stage.
I wouldnt worry too much about the fitness - I think, given a bit of hard work, you have enough time to get fit enough by January. Even then if you are still a bloater and can't hack it they will be more than happy to put you on remedial PT !! Just dont eat too many mince pies at Christmas.
Good Luck
Oggin