As long as you're being cognizant of overtraining and taking the necessary rest intervals for your body, you can't have too much base. ..it'll just help you become stronger :)

The main items I would keep in mind are rest, and possibly some fun races/events somewhere in there to help break things up a bit - C priority type races/events.

I spent 10 months doing base training for IMAZ, best training plan I could have ever done. You can never do enough base, just don't confuse base with easy :).

No weights. Every available training moment was either swim/bike/run...well, lot's of bike, and stretching everyday, Mondays off.
Here's the problem with Zones. First, they are differenet for the bike and run, run being higher HR. My opinion, always train to the higher HR, so use the same training zone for both. The zone itself doesn't need be the exact Friel defined Z2, better it be a 10bpm pace that you can work where the high end is a soild effort. I think the error a lot of people make is that they allow their zone workouts to sort of languish in the low ends and figure, hey, it's base so who cares? You need to push the limits a bit. I would frequently push a little beyond my perscribed 138-148 HR during training when climbing hills for example just to acclimate my body to providing a higher output of work when called upon. Or doing a run at the absolute top of the zone and holding it there. Sometimes we are afraid to push because we think we will go anerobic and that ruins the base training, but that level is well above Z2, so go aghead and push the pace after a while.:)

FWIW here's a snapshot of my training for IMWI showing HR zones. Note that I am out of zone 2 a lot, and even sometimes into 4 and 5. I try to keep my trips into 4&5 brief, but living with a lot of hills it's hard to do. What I"ve tried to work on is getting my body to recover quickly so that if I do go above say zone 2 or 3 going up a hill that I can recover right away and continue on in the prescribed HR zone for that day. I figure it will suit me well on a hilly course.

I love it.
I was on training peaks using their virtual coach over the winter. You plug in a bunch of info and it builds an anual training plan with your a, b and c races. I then switched to Performance Training Systems for coaching and they set you up with a training peaks account as part of your package. So now every month the coach looks at what you did last month and what you have coming up then they put your workouts into Training Peaks. You do them, then log them.

Or, you can buy one of the pre-built plans from them as you mentioned.

It's a really easy program to use and the weekly summaries and reports and such are nice so that you can look back easily and see what you did that week or how you were performing.

Just watch how you answer the questions. I put in the ideal training I wanted and it generated an anual training plan for me that had me starting at 20 hour weeks 11 months from IMWI and building from there. Had I not had a job, wife, kids etc...that would have been ideal, but it is why I ditched it for the coached method.