It can strengthen your non-running muscles and rests the ones you use during your runs. You will be able to focus on specific muscles, such as inner thighs, that do not get trained as much during your runs.
By balancing your muscles, you also reduce your chance of getting injured. Low impact cross exercises, such as swimming, can also lessen the pressures and the stress on your joints.
Cross exercises are also great to improve the level of your cardiovascular fitness and this will benefit you during grueling distance running races.
Running everyday will burn out everyone, even the most devoted running enthusiast. Cross exercises provide ways for runners to have that mental break from running. By taking some time off from running to exercise in other activities can help you get that excitement back when you return to running.
Runners suffering from injuries are often told by their doctors to stop running temporarily so that they can recover. However, if you are doing cross exercises, you will still be able to practice even if you are injured. By doing this, you will be able to maintain your fitness level.
The answer to this question is entirely up to you and your expertise level. If you are a recreational runner, then you can try to cross train for between 2 and 3 days per week. If you are an experienced runner and love to run races regularly, then you can try doing light and easy cross exercises during your rest days. Cross training is also a great alternative for runners who are traveling and have no access to tracks or a treadmill.