newbie from the uk

So one year to a full Ironman and/or 4 months to a half Ironman.  I guess my first question to assess fitness is, how long have you been doing the 40-60 mile rides?  

Those HIT sessions, is it essentially an hour on the treadmill?  How long have you been doing those?  Any road/trail running?

And swimming, I take it you are starting from scratch on this?

I think you have the fitness to do the HIM in Bahrain in December since you have been doing these volumes for months now and have a swimming background.  You essentially have the bike fitness (are you only doing bike training once per week?).  You have the swim fitness, just doing a new stroke - focus on that and doing the distance.  Running, well, you may be short there.  You may be able to build up to 6-8 miles safely by December - a full 13 miles from a 5k may be a stretch.  I would say you can do it with the caveat: don't rush the run as that will get you injured.  Safely build up using the 10% rule every 2-3 weeks followed by a 'rest' week.  If you can't get to near 13 miles on race day...that's fine, plan to walk.   And this first race, don't 'race it', 'train it'. 

No reason you can't do a HIM or IM as your first race.  

Both are about training more than about racing.  On average, about 93-96% of the people who start an IM race officially finish it.  The 4-7% who don't include tho pro's who drop out for season strategies, a few who get injured during the race, and the ones who don't make the cutoffs.  But...if you can start, you're likely to finish, especially if your race strategy is supported by your training.  What isn't published is the number of people who sign up but don't even start the race.  This is generally a much higher number (10% or so at a minimum).  These are people who sign up and get injured during the training year, don't train enough to want to participate on race day, have other commitments that interfere with the race.