Why Maintaining Your Speed Is Important
Experienced marathoners know the key components to race day success are a high aerobic threshold (the fastest pace you can run while still staying in the aerobic zone, which closely correlates to marathon pace) and the ability to burn fat as a fuel source. The most effective way to improve these two things is a steady diet of aerobic mileage and threshold work.
Unfortunately, these two types of training don’t directly improve your speed. That means if this is the only type of training you’re doing, you can go months neglecting an important energy system during marathon training.
Neglecting any energy system for a long time period is detrimental to your overall fitness. In particular, ignoring speed work during the marathon base building period reduces running efficiency. Speed work increases muscle strength and power in your legs, which improves economy, or how much energy it takes to run at a certain speed. When muscles become stronger, fewer muscle fibers need to contract to hit a particular pace. Thus the energy expenditure is lower, which can make running at marathon pace feel easier.