Sport scientists refer to a period of lesser training, or no training at all, as detraining. In recent years there has been an increased interest in studying and understanding the effects of detraining, and how to prevent significant declines in fitness during periods of inactivity. Much of this research has come from clinical studies of people recovering from surgery and heart attacks, while some involved athletes recovering from major injuries. Your concern appears to be with a possible decline in your level of aerobic, or cardiovascular fitness, as opposed to a loss of muscular strength and power, so my response will for the most part refer to detraining as it relates to aerobic fitness.
Regular endurance training of sufficient intensity, such as training for a half marathon, leads to two major adaptations in the body: improved ability of the cardiovascular system to deliver oxygen to the working muscles, and improved ability of the slow-twitch muscle fibers specifically used for aerobic exercise (running) to utilize the oxygen. These changes to the body’s oxygen transport system enable you to run longer and at a greater pace as you become more aerobically fit. Research on detraining shows that when training is stopped entirely for a relatively brief period, for example a few days, the cardiovascular system detrains rather slowly. In other words, you won’t lose much fitness by taking a few days off. As one might expect however, extended periods of inactivity can lead to significant declines in endurance capacity. Studies do show that losses in cardiovascular fitness are significantly greater than reductions in strength and power over the same period of detraining. I assume you do not plan to stop exercising entirely but are merely going to cut back on the volume of your workouts. If that is the case, then you should be able to maintain your present level of cardiovascular fitness with a lesser amount of exercise if you consider the following.
Three exercise variables, frequency, intensity and time (duration), described by the acronym FIT, play critical roles in the development and maintenance of aerobic fitness. Proper manipulation of these factors during a period of reduced physical activity can prevent significant losses in VO2 max, defined as the highest rate of oxygen consumption attainable during maximal exercise. VO2 max is generally accepted as the standard measure for aerobic fitness Current research suggests that an athlete can maintain a pre-detraining level of aerobic endurance if he or she (1) reduces the frequency of training, say from 6 days to 4 days per week, but still maintains the intensity and duration of workouts, or (2) reduces the duration of workouts, say from 50 to 40 minutes, but still maintains the frequency and intensity of training. In both cases it appears that the most critical exercise factor impacting aerobic capacity during a period of detraining is the intensity of exercise. Studies show that a reduction in training intensity by 1⁄3 or greater results in significant losses in aerobic fitness, even when frequency and duration of workouts remain constant. Physiologists Jack Wilmore and David Costill, authors of “Physiology of Sport and Exercise,” maintain that aerobic training must be conducted at least three times per week at 70% or greater of VO2 max in order to prevent declines in cardiovascular endurance. In short, if you maintain the intensity of your workouts you should be able to run fewer miles per week over the next couple of months and still maintain your present level of cardiovascular fitness. In this case the old coaching adage “it is easier to stay in shape than it is to get in shape” rings true.
You can monitor exercise intensity by checking your heart rate during the workout, as heart rate is highly correlated with the amount of physical work being performed. To do so, stop running for a moment and locate your pulse by resting the index and middle finger at the base of the wrist, or at the side of the neck near to your Adam’s apple. Count the beats for 15 seconds and then multiply by four to get beats per minute. The higher your heart rate, the greater the intensity of the activity.
Also keep in mind that during a prolonged period of detraining you will be burning fewer calories than normal and as such will probably have to reduce calorie intake to match your activity levels. Because VO2max is calculated per unit of body weight, your measured level of aerobic fitness will automatically decline as your body weight increases.