This depends on your training history and current fitness. The more specifically fit you are the less transfer these flat runs will have to your event preparation. So if you are quite unfit, then any and all continuous aerobic exercise you do will increase your basic aerobic capacity. The more fit you are in terms of your event, the less benefit these flat runs will transfer over to your event. You might recall a passage in our book where we mention that; At Steve’s fitness level, exercise like swimming or paddling will only be a good recovery workout as it will not be able to increase his even specific fitness.
These flat runs can serve as basic aerobic support for sure. Accumulating more aerobic volume, especially running is never a bad thing. However as you become fitter or get closer to your event more and more of the training has to take on the characteristics of the event itself. How much does flat running look like front pointing up the Japanese couloir at the base of the Cassin Ridge? Not much, right? Climbing steeply uphill hill requires more and different muscle groups than running on the flats. Even in the base phase and even if you are not very fit, I would encourage you to spend most of your basic aerobic training on steeper terrain while using the flat runs for recovery workouts.