More on what happens when we train
Enduro cycling is interesting. It's a real mish-mash of requirements, unlike other endurance sports it has a (sometimes significant) over threshold power component. Many races end in sprint finishes, the nature of the sport is such that riders can't just ride at their threshold power output and expect to win in anything except a time trial. Attacks, short hills, sprint finishes etc mean the endurance road racer must have a mixed bag of adaptations to training. Road racing and MTB racing is primarily aerobic in nature, but not entirely.
What about long, slow distance? Traditional base miles, go out and ride for 1,000km before doing any intensity? What's the stressor that that introduces? Energy use. The stress of the body running low on fuel forces it to adapt to burn more fat. Not as a weight loss thing but as an energy source.
Getting back to Seyle's adaptation syndrome, it boils down to this, thanks to Lon Kilgore for the concise summation :
The crux of the correct application of Selye’s theory is to understand that a disruption of homeostasis must occur in the physiological system of interest in order for adaptation and fitness improvement to occur in that same system.
So, roadies and MTB racers doing long races need to stress the body by running it low on energy reserves. This doesn't boost VO2max, but it does force an adaptation to metabolise more energy from stored fat and an adaptation to store more glygogen. Lon writes it well again :
Long-slow-distance training is energy substrate depleting in nature. It has been shown many times over that glycogen stores can be totally depleted with this type of training, and depletion of an energy substrate should be considered a fairly significant homeostatic disruption of metabolism. It would not be prudent to consider only complete depletion as a disruptive stress, partial depletions should be considered disruptive as well IF AND ONLY IF the depletion is larger than previously experienced by the trainee. This type of training can also exceed the body’s ability to metabolize fat for energy. Driving a metabolic system beyond its normal range of operation or to failure is definitely a disruption of homeostasis. Combined, the stress of glycogen depletion below normally experienced levels while simultaneously exceeding fat metabolic capacity drives an improvement in storing and utilizing these two energetic substrates and results in being able to run longer – thus endurance has improved but VO2max has not improved.
So, LSD has a place. It's probably not as important as increasing aerobic capacity is for roadies and other enduro cyclists, but it is important. How much needs to be done? That's an interesting question. The balance between high intensity intervals (E3 and above) that stress the aerobic system and energy depleting training is a challenge to get right and varies from athlete to athlete. Long, slow miles gets a rider better at riding long, slow miles. We also need to boost aerobic capacity by doing high intensity work. Something that really stresses the aerobic capacity of a rider, for example riding at just above a sustainable power output is good for raising thresholds, as is doing shorter, high intensity riding with limited recovery, such as 1 minute ITT efforts with 1 minute's recovery between efforts. These are spectacularly good at driving increases in aerobic capacity. If all you do is base miles, all you get good at is base miles. Intervals are vital to driving up your performance as an enduro cyclist and it's possible to use your time more effectively than by rolling around for 8 hours on a weekend ride. E3 and to a lesser extent E2 riding is very energy depleting, longer blocks of E3 will force the body to run low on fuel, and adapt to the need for more without spending unsustainable amounts of time on the bike.