Many runners, run for years without ever doing any other type of exercise, and some of them get along just fine, but I think every runner can benefit from strength training. In my opinion, every runner should strength train. We now know that cardio is not sufficient. We lose muscle mass with every year, and running is not sufficient to get that muscle mass back, not by a long shot.
Whether you work out at a gym on machines, with free weights, or with body weight, strengthening all the muscles in your body is essential. I've always believed that, but in recent years, much has been written and much research has proven that strengthening muscles is just as important as cardio exercise.
Smart runners will make strength training an integral part of their training regimen.
Consider the value of strong muscles. Strong muscles stabilize joints, and they take pressure off of bones and joints, thereby guarding against injury. Strong muscles can do more.
Strong muscles lead to more controlled movement and improved balance. That means fewer falls and more saves. Have you ever slipped, saved yourself, and then thought, "What if I had hit the ground?"
Have you ever thought, "Wow, that was lucky!" Maybe luck has nothing to do with it. If your muscles are strong, you will be less likely to experience injuries.
Strength training alters the proportion of fat to muscle. Everyone would do better to have a higher proportion of muscles to fat. Right?
Stronger muscles enable you to run with better form, too, which will be faster and easier on your body.
Running doesn't use all your muscles, it doesn't even use your leg muscles equally, and it doesn't use them through their full range of motion. Many injuries are caused by muscle imbalance, and if your muscles are strong only in a portion of their range of motion, and weak the rest of the movement, that is a recipe for trouble.
So, get to the gym or just start some intense body weight training at home, but whatever you do, make sure you work all your muscles through their full range of movement.
A caution here, though. Never strength train during the week prior to a race. That's right. Don't worry, you won't lose any muscles that quickly. The main thing is to make sure your legs are in tip top shape, completely fresh on race day.