With the help of flashy marketing and cherry picked supporting studies whose results are extrapolated a little beyond reason, a handful of cross-training systems have come to the forefront of fitness recently. Is this a major problem? Nah, not really. These things help people get healthy and fit, and they can result in significant changes in the body over a relatively short time. That's a good thing.
The problem I have is that some of these systems appear to hint or imply that any other exercise system is inferior, and that their approach to programming their high-intensity exercise sessions somehow better optimizes fitness than other systems of training. These cross-training systems are great at keeping people on track and working as hard as they should if they want to improve their strength, muscle tone, and anaerobic capacity. Again, a very good thing.
The problem comes when people take some of the cherry picked evidence used as "scientific" support (there's nothing scientific about cherry-picked data), to mean that instead of aerobic training, we need only do high-intensity interval training to reach peak fitness.
The simply reality is that peak performance is the product of an optimal mix of aerobic and anaerobic (interval) training. If you do one without the other you develop only the aerobic or anaerobic components of your muscle's energy systems. If all you ever do is aerobic running, you plateau, and often in a very interesting way.
Cyclists often train at aerobic intensity for long periods of time, and those that use heart rate monitors begin to notice after awhile that they get locked into heart rate zone 3, and find it very difficult to find the energy to get up into zone 4. What they've done is developed their anaerobic systems about as much as possible at their current training load, while their anaerobic systems and perhaps some of their fast-twitch muscle fiber strength is relatively undeveloped. Coaches know the way to get a cyclist through this plateau is through a combination of strength training and high-intensity intervals and tempo efforts.
Likewise, 100-400m sprinters are not typically great marathoners. Their training usually doesn't include the long aerobic training runs that middle distance runners often do, and which long distance runners ALWAYS do. These short sprints don't require much in the way of aerobic energy. They are high-intensity, largely anaerobic efforts, so sprinters usually train to maximize their anaerobic energy systems and fast-twitch strength.
Through trial and error, coaches and athletes have learned that 10-20% of weekly miles should be in intervals and tempo workouts, with the rest of training mileage being comprised of easy recovery runs, and moderate-intensity long aerobic runs. Training for runners interested in improving their performance therefore involves working out at a variety of intensities over a variety of distances. So, when branded cross-training systems out there imply that distance runners are behind the times and should embrace intervals and variety in their workouts, they are half right. Distance runners certainly need some high-intensity interval work to optimize performance, but they have actually known the value of intervals for decades, and the training schedule includes plenty of variety, including intervals and strength training. Indeed, these types of diverse training schedules in running pre-date every one of the branded cross-training plans on the market.
If intervals are the way of the future, it seems that these cross-training systems have brought us back to it. Back to the future. Yet, their memories are so short. Of course, I don't intend to demean intervals and cross-training. As I've said, it's a good thing. I think we should all do some amount of cross training and we should include workouts of varying intensity in training for our primary sport. Just how much we should do depends on what we are trying to achieve, however.
If someone is telling you they have a new method working out, it's probably been done before. If they tell you they have a better or more efficient way... well, they're probably trying to sell you something. Everything has been tried and the evidence collectively suggests that there are no clear shortcuts to optimal performance in long distance running.