Thanks for writing into the forums with this valuable question. Based on what you are saying here’s what I recommend you try.
Since in both “studies” you were struggling to complete the longer workouts it is clear your fat adaptation was not sufficiently well developed to support those efforts. From a metabolic standpoint this means that glycogen was still contributing enough to the overall energy needs that your glycogen tank ran low during these long workouts. Your reduced HR and overall work capacity 4000ft vs 10,000ft days indicates exactly this same thing. You are just not at the level to support that much work on on only fat.
Add more midweek fasted aerobic workouts and use shorter fasted workouts on the weekends instead of trying to recreate a 6 hour day that you can do on carbs the bonking and need to eat mid workout. Try 2-3 hours and build on that. While a full keto diet will have a dramatic effect on fat adaptation, as you found out it can also have a dramatic effect on your training until you are fully fat adapted AND well trained That is why we propose a more moderate diet modification and training mixture to produce the desired effects for both fitness and fat adaptation over the several months. We’ve seen big shifts in as few in 4 months using this method (Adrian Ballinger training 20+ hours/week) but normally we smaller changes just starting to occur 2-3 months in and significant changes 6 months along.
I hope this helps,