ironman base building

I'm no expert on this topic (THAT's an understatement) but if you're referring to your "granny" gear - the smallest of 3 front chainrings- I would say you should not be using it during training or at least very minimally.

I have a 2 hour Spinerval dvd, and he says at the beginning that the granny gear will NOT be used.

If you only have 2 front chainrings, then you would mix up the training between the 2.

Alot depends on what kind of base you have and what limiters you face?  Small ring and big ring rides build up different kinds of muscular systems.

Riding in the small chain ring is recommended by alot of coaches for early base riding.  Including Gordo. 

But I think as you progress, you might want to do some rides or at least a portion of some rides with the big chain ring.  Like maybe 20 mins S/C, 20 mins B/C, 20mins S/C?? 

Some of the coaches here might have a better explaination for you...

I agree that base building depends on your limiters and current level of fitness.

I'm currently in the midst of my IM base building and I'm doing strength work (big ring lower RPM work) now to prepare me for the rolling hills of IMWI....but biking has always been my strongest leg.