Help!

i did my first IM in april 09 in 12:21 using the 20 week programme.
Temperatures were in in the mid 90s but I managed it.

I did little until the start of His year when  i planned to do Coeur d'Alene at the end of June with St.Croix 70.3 along the way.
I have been using the 17 week intermediate programme and it was going reasonably well.
Until St.Croix!
The heat and the hills flattened me. My tank was empty before the end of the bike and I really struggled on the run with lots of walking. I completed in 6:46 having hoped to beat 6 hrs.

I did nothing in the past week. I was on vacation and it would have been unfair on my wife. In any case I'm not sure I could have managed to do much in the carribean humidity and heat beautiful and all as it is.

I am now back home in Ireland where my training to date was done in 10-50 degree low humidity or indoors (with heating on and no fans to try to get used to termperature.

My dilemma: If I struggled that much in the HIM am I wasting my time trying a full at the end of June?
I have not done any 100 milers yet and running is at 13 miles. I am comfortable swimming.
It looked like the ideal time to do a HIM , 8 weeks in advance.

Is CdA hot and humid? I know its hilly.
Am I just not fit enough or do i need to just kick myself in the butt and get on with it?