I ran the New York marathon in 1984. I trained tirelessly in the months and weeks leading up to it.

Sure, I was elated about the challenge, but I was exhausted. I developed a dry cough and got little sleep. My muscles ached around the clock. I had blackened toes, dehydration, and an urge to eat everything in sight.

At the time, I wondered if my elevated exercise regimen had any beneficial effect on my immune system. I seemed to be experiencing the opposite.

The short answer is that moderate exercise strengthens your immune system. It results in a rise of white cells and a redistribution of cells between the blood compartment and the lymph nodes and other peripheral tissues in organs like the liver, lung, and kidney.

But recent research has shown that prolonged periods of intensive exercise like I was doing while training are not necessarily good for you and can depress immunity. That’s right, the profound stress induced with extreme athletic activities can be a downer from a biological perspective.

In other words, don’t stress yourself out doing something that is supposed to help you not stress out.

Prolonged exercise may impair T cell responses, natural killer cells, and white cell functions. Cytokine balances are also changed with prolonged stressful exercises and immune responses to primary antigen exposure. This may lead to alterations in what we called mucosal immunity and why the elite runners frequently talk about symptoms associated with upper respiratory tract infections during periods of heavy training and competition.

Mucosal immunity, or the immunity around the mouth, nose, vagina, rectum, and the entirety of the GI tract, is the border patrol of your biological soul. Mucosal immunity even has its own immunoglobulin or antibody class called secretory IgA and intense periods of exercise lower secretory IgA antibodies.

Single short bouts of what I call feel-good exercise, however, result in endorphin release. What I mean by this is a workout that gets your heart rate up but is not super stressful — nothing that pushes your body to its limits of endurance, especially when you have not been exercising regularly.

This kind of aerobic exercise like swimming, running three to six miles per day, or just walking for one to two miles daily can improve immunity through reductions of inflammation, maintenance of thymus gland size (when you are younger), and alteration in the composition of older versus younger immune cells. It also enhances immune surveillance or the process by which your immune system cells identify, monitor and destroy infected, cancerous, or precancerous cells, which is essential to our understanding of aging and the decline of immune function.

Psychological stress in people who exercise is also ameliorated for reasons that are not totally clear, but probably as a result of mindfulness that I spoke of previously and those endorphins that provide feelings of pleasure. It’s a kind of mobile meditation that most runners know as a “runner’s high.”

The important thing to understand is that extreme is bad, especially as we age.