OK, I’ll admit I took your post a little out of context to make a point. But in my defence you didn’t really provide any context, you just converted the total calorie count of a 90 minute session - and a particularly tough one at that - into it’s equivalent in gels, bananas, etc, you didn’t say how many of those calories you thought he should be taking on during the workout.
I stand by my point that being unable to complete a 90 minute workout 3 hours after eating a meal is almost certainly not a fuelling issue, and therefore most of the advice in this thread regarding fuelling is missing the point. If he was struggling first thing in the morning having not eaten for 12 hours it would be a fuelling issue. If he was struggling to complete a 2-3 hour workout on just water it would be a fuelling issue. If he was getting through the workout but then feeling tired all day and struggling the next day it would be a fuelling issue, or at least a post-workout nutrition issue. 90 minute session starting with fully topped up glycogen reserves? Should be very doable. Even using your Elephants+4 example (and I’d recommend starting on a much easier 90 minute session, the Antelope suggestion above is a good one), around half of those calories will be coming from fat, so there’s maybe 600 calories coming from carbs, and a 95kg guy who has just eaten should have 2000+ calories of glycogen stores to draw on, probably a fair bit more. So he shouldn’t be even close to running the tank empty.
Is it a good idea to take on some calories during longer, more intense sessions to aid in both performance and recovery? Absolutely. Is that what’s stopping the OP from completing a 90 minute workout? Highly unlikely.
Antelope should be doable, if you’re struggling with that then start out with an easier variant like Antelope -1 or -3 and build up from there. Or go easier still and do an Endurance workout like Andrews or Holt Hill. And you really shouldn’t need a few days rest before a sweet spot session - whole point of sweet spot is that it’s an intensity which is hard enough that to trigger adaptations but without being so demanding that you can’t go again the next day.
If you haven’t been riding over an hour, then I also think it’s highly likely that your FTP is set too high for longer workouts. Rough definition of FTP is the power you can sustain for an hour. Ramp test estimates your FTP from much shorter efforts, but an underlying assumption of the test is that the rider has a good balance of aerobic and anaerobic fitness. A rider who tilts more to the endurance side (e.g. an audax rider who does a lot of very long rides mostly in Z1-2) will likely get a low FTP estimate from this test. A rider who tilts more to the anaerobic side (e.g. a crit racer who does lots of short, hard intervals in Z4-6), will likely get a high FTP estimate from this test. Your training means you’re likely tilted more towards the anaerobic side at the moment, so your FTP setting may be fine for shorter sessions but it’s likely too high for longer sessions where your lack of endurance will start to count against you.
I’ve scheduled in Antelope in a couple of weeks time. Generally I ride Sunday, Tuesday, Friday. but to try and add extra volume I am going to add a steadier ride in on a Monday, much like the Wednesday workouts from mid volume and then on a Friday do a 90 minute workout instead of my normal 60.
My FTP is from the 8 min test as I test really low on the Ramp test. With the 8 min test I have never not been able to complete a 60 min workout and never had to back pedal. I am fairly confident the test gives be a good number to work from.