Get Joe Friel’s Triathlete’s Training Bible for some excellent ideas. I just finished IM Florida and did virtually nothing but high volume in my bike and run training (ie. almost no speedwork) and just swam with my regular group until about 7 weeks out when I started doing workouts with long repetitions. It resulted in 9:27. I think that ironman is such as strength biased event that just cranking up the miles and putting in as much time as you can spare is the best approach for success. In the past, when I focused on quality, I fell apart on the run (which is my strongest section) and couldn’t use any of the speed that I had. This year I’m focusing on Hawaii so my prep will be to get a reasonably solid base through the winter 15 or 16 hours a week starting in January, then to focus on speed for May and June while keeping around 15 hours each week, then to crank up the volume to 24 or 25 hours a week through July and August and early September with no speedwork other than a few race days which I’ll train through. Then start to taper 3 weeks out bringing the volume down to 15 hours, 10 hours, 5 hours then race. I never schedule days off but take them whenever I feel too sluggish to train. This is what I’ve found works best for me personally while others that I know seem to do fine with low volume and making sure that they take a day off every week whether they feel the need or not. Your best bet is take your own experiences and do research reading like the training bible or some online coaching programs (I’ve looked at some like the Roch Frey/Paul Huddle program a friend of mine ordered a few years back and I try to brick up my bike/runs whenever I can like they advocate) or to hire a coach and put all your faith in their knowledge to get you through. Best of luck.
Sorry, but your questions are waaay to complex to answer in a single post (heck, even in multiple posts - that is why they write books on this matter).
Agree with the other posters about Going Long and Training Plans for Multisport Athletes. The Triathletes Training Bible was way too confusing for me. You could also get a copy of Gale Bernhardt’s 13 weeks to a sub 13hr IM training plan for guidance (you can have triathlete magazine fax it to you). I used it to go 12 and change at IMFL. It’s great for a beginner/intermediate but for for advanced triathletes I’d recommend Going Long or a coach.