The concept of training frequency played a major role in the physiological development of our ancestors. Archeologists have found that ancient humanoids living in warm climates performed physical activity like hunting and building several times a month. This is just another way of saying they performed exercise on a regular basis. In comparison, those that lived in colder climates had less of a chance to hunt and build on a frequent basis due to harsh weather conditions. This lack of frequency in physical activity for cold climate humanoids led to weaker genetic evolution and shorter life spans.
Fast forward thousands of years to present day. Exercise physiologists and scientists alike all agree that out of the four training variables (Frequency, Intensity, Time, Type) Frequency is the most important. As a species, humans are genetically engineered to be more receptive to multiple bouts of physical activity (hunting, building, scavenging) spaced out over a set amount of time. For example, say we have two ancient hunters performing a total of 16 hours of physical activity over the course of a month. One hunter performs 2 hours of activity two days a week. The other hunter performs 4 hours of activity one day a week. Based on historical and current evidence, greater adaptation (both short and long term) will occur in the hunter with greater frequency in activity, regardless of how long the activity lasts. In other words, take the marathon like sessions you have planned for your athletes and try to break them up into separate sessions (two 1-hour sessions in a day rather than one 2-hour session).
As a performance coach, you should always be looking to formulate the best plan. Therefore, the first piece of the puzzle that should be addressed is frequency of training.