19 weeks out and not trusting my IM plan

What are your concerns with the plan?

Are you saying that there is only one 6 hour ride in the whole plan?  The one thing that I found reassuring for my IM was that I had cycled over 90 miles 6 times, including 3 over 100 miles. Is this a plan with a coach?  If so, bring up your concerns with your coach.  If not, there is nothing keeping you from adding some miles onto some of your rides.

Successful IM is not based on one type of workout or one workout but the sum of the whole of all your training.

There are different views on the long bike in training for IM. Some plans/coaches have lots of long rides and other have fewer.  For my first IM I wanted to know I could do the distance and made sure my coach had me ride close to the distance not necessary for racing but often for the mental belief you can do it. Most of my other IMs I would do some rides in the 80-95 mile range but no longer. I find that once you can ride for 5 hours, 6 or 7 is not that much different. I find the 3-4 hour rides that are at harder effort of IM key to race success.

Those long swims, rides and runs are often key mental milestones for first time IM athletes by helping you gain confidence that yes you can do it.

If you doubt your plan and it will be a lingering concern now is the time to change. Sure you can add time or distance to your long rides later in your plan but what else are you going to change. For me trusting the plan or the coach is key and mucking around with it means you don't trust it.

As Kathy noted, IM training is all about the accumulated stress of your training and not one singular workout.

I've done a half dozen IMs.  Only time I ever did a training ride over 4:30 (and most are in the 4:00-4:15 range, 80-85 miles) was in the build-up to Ironman Canada last year because the Whistler hills kind of scared this flatlander.  I drove out somewhere to ride some hills for more specific training but otherwise wouldn't bother with it.

My IM bike splits have been 5:40 for my first IM a few years ago to just a shade over 5 hours this past year.  Average around 5:15-ish.

You can create the same training stress in a ride of less time by upping the intensity a bit.  Not sure if you have a power meter or not but a simple TSS calculation would give you something like this:

5 hour ride and 70% equals 245 TSS

4.5 hour ride at 73.8% equals 245 TSS

4 hour ride at 78.3% equals 245 TSS

So I can accomplish the same training stress in one hour less time by riding 8% "harder."  I'd prefer less time but more intensity.

How many rides and how many hours are you riding per week?

It isn't uncommon to question the training plan and whether you're doing enough or not. 

I'm not familiar with your specific plan, but as long as you get a couple high-quality 5-6 hour rides in, you'll do OK in the race if you can keep your effort in check.  I think it is far more common for athletes to do too many 6-hour rides, and they end up injured or burned out.  By the end of training, they do not want to see their bike ever again, and yet they have a 6-hour date they can't cancel. 

I've done the BT Intermediate IM plan (loosely followed...) and it alternates long bike with long run weekends.  As such, there aren't a lot of 6-hour rides. 

From experience, it both is and isn't. There isn't really much more you're going to pick up by going longer at once, but the rides are longer so do take more out of you. And in that sense they can compromise your overall training due to the additional recovery needed.

that would be the response on SlowTwitch.  

^^^More or less this^^^

IMO, this demonstrates the importance of training at an appropriate intensity.  Threads about "am I training enough hours or miles?" are common, but threads asking "am I training at the right intensity?" are less common.  As implied above, a 4 hour ride that challenges you is usually going to be more beneficial than tooling along soft pedaling for 6 or 7 hours.

I like Gale's plans for the average AGer looking to finish upright with a smile.  Unless you have another reason to question the appropriateness of the plan for you, such as struggling to complete sessions, an injury, or every session feels ridiculously easy, stick with the plan.