IM Bike training - thoughts on 100% interval training

I've done endurance cycling for years & IMHO most still need some long rides to earn those butt callouses.  Most winters my bike work is hard trainer rides w/reg intervals, but usu <60-90min sessions (for sanity reasons).  On that 1st spring century the legs are usu OK (though not my peak form), but the ride is still a PITA (literally).  Seems to take a few ~100+k rides to get comfortable in the saddle again.

Not saying intervals are not good, but I think a few long slower (aerobic) rides are useful for most to re-acclimatize to distance cycling.  I've read somewhere that the trend among IM elites is towards shorter more intense bike training sessions as more efficient use of training time.  However those athletes have major experience in long bike legs (pacing/hydration/nutrition/equipment/etc.).

I'm curious what kind of 'high intensity' riding you are doing for 5-6hrs.

In general, I think lots of intervals on the bike are fine.  But in the last several weeks, you would ideally look to get more race-specific.  Which means LONG, steady intervals (IM intensity or a little higher), staying aero, etc.  Especially if you are planning on 5-6hrs of riding already.

Well, most of the Spinerval sets (at least on the DVDs I have) fall into several buckets..as examples:

1. Example 1: (on the longer DVDs) 30 minute set...first 10 minutes, alternate 1 minute with big chain on front, 15 on back...then 1 minute small chain ring on front, 15 on back (higher cadence)....next ten minutes, big chain ring on front, 15 on back...alternate in saddle one minute, out of saddle on next.  After 1 10 minutes, soft pedal for two minutes before going to next ten minutes.

2. Example 2:  10 minute set...random gear settings every 30 seconds called out by Troy (big chain ring + 15 on back...then to small chain ring with 18 on back, etc.)...after ten minutes, soft pedal for a minute. 

So, when I said 'high intensity' I meant I tend to push it harder in these workouts than I would on a long, steady interval on the trainer...I just find it easier (less mind draining) to keep the intensity using the spinerval DVDs then a more steady, long ride.... but was sort of worried about what you point out....this approach is not as race-specific as the long, steady intervals.

I'm really overthinking this, I know. 

I rode a 6:14 at CdA in my first IM based on 80% of my rides being on the trainer.   Most of those sessions were in the 1-2 hour range at varying intensities.  Averaged around 150 miles a week.  Literally zero hill training.  I did however ride at least 6 days a week, and often 7.  FWIW I had less than 6 months bike base before my IM training started.

For AZ in November 100% of my training has been on the trainer.  Same formula:  6-7 sessions a week, three being 2 hour rides, the rest 1 hour rides.   I'm feeling stronger and fitter than ever and hope to go well under 6 hours on the bike.