Great post, i think most athletes struggle with this because its difficult to understand that “fitness” is truly only measurable to a single sport as far as sports specificity. It doesnt matter how fit you were in whatever sport. Another thing people discount is the complexity and specificity of movement. For instance, i went through a similar change from road running to mountain running. Now i can do mountain runs all day with little or no calories and not be destroyed the next day, however, my road running efficiency has become abysmal in the sense that now my z3 pace on the road is 1 minute to 1:30 slower per mile. In other words, you cant be good at everything all the time, and developing efficiency in a certain style of running takes time and often at the cost of efficiency in other sports or modalities. People think that they can just go out and “run” but running is a skill, like weightlifting, climbing etc. Skills take time to develop and refine. Most people can move their legs at a pace faster that walking-but i would argue that not many people who consider themselves runners actually have developed the efficiency and skill of movement to actually be called “runners.” Many people take years to refine their technique, so it is not surprising that after biking a lot at a high level you do not have refined running technique and therefore do not have the efficiency within the skill to maintain steady cardiac output (amongst the other variables such as nutrition, training stress, sleep etc).