Big Day Training
Search "Big Day Training" on my board ( The Tri Forum for more on big day training. This is a far more effective long workout for IM that won't result in extended recovery.
Swim 60 mins, Bike 5 hours, Run 60 mins
Once you get to the point that this is easy then you can make the back half of each session steady-state. This type of workout addresses the key limiter of most IM athletes -- base endurance.
The next step (with the workout that I posted above) is to increase the steady-state component of the training. An example of what I might do -- I'll speak my own lingo...
I know very few athletes where endurance is not a limiter, it requires constant work. Most IMers think that they graduate to tempo-type training (I certainly did when I started). It doesn't work that way. The core of the optimal program is a varying amount of steady-state training, year-round.
The ultimate goal of training is to improve race performance at goal race intensity. Most of the IM field has an average race intensity of AeT, or lower. Many (most?) of the field believe that IM performance is optimized through maximizing pace/performance/endurance at LT or AeT+15/20. That's not my experience.
If you test aerobic efficiency of most endurance athletes -- lactate step test or other method -- then you'll most often find upside through increased focus on steady-state efficiency. This is a message that many don't want to hear. Why? Because it takes a long time to get good and we're all in a hurry.
Even elite and top AGers tend to race at average intensities that are lower than what many realize. True ironspeed is achieved from superior pace, power and endurance around AeT. Take my numbers when well-trained... AeT Power about 265w, AeT Pace just under 4 min per K. For a decent single-sporter these are pedestrian performances -- until you seek to maintain for 180K and 42K respectively.