The first thing to establish before we get into guidelines and protocol for cross training. This is whether it is beneficial for you personally to add cross training into your running training. What I often witness is athletes adding in cross training for the wrong reasons. Sometimes it to lose weight. Sometimes it is to release frustration. And other times it is just for junk over training that they believe will help them in the long run. I will run through situations where cross training can be helpful, which cross training activities to engage in, and some good workouts that can be done.
Lets assume first that you are considering cross training to keep your fitness as you have an injury that has limited or eliminated running all together. The concern here is not whether or not cross training will push you into the realm of over training as it is very difficult to over train with cross training. The concern here is whether the cross training will prolong your healing process of your injury and keep you away from running longer. If there is any possibility that the cross training activity you choose may hurt your injury further, you should avoid that activity at all costs. Remember, the point of cross training is to improve your running, and if it is what is preventing you from running that is a problem. Usually however, between biking, elliptical, pool running, and lap swimming you should be able to find an activity that does not work.
But how should you choose between all these options? While you should factor in which activity you enjoy more as cross training is hard to stick to, even if you like the cross training activity you are doing. Also important is to choose the activity that is most similar to running, and obviously, does not aggravate the injury. If you have access to an alter G treadmill or an underwater treadmill these are the best options as they are very similar to running without the impact. If these are not an option, the most running like activities, in this order, are pool running, elliptical, bike, and finally laps.
If you are not running at you can train up to 2 hours of any combination of these activities everyday. I realize most of you did not run for 2 hours when you were healthy but when it comes to cross training, you need to do much more in order to attain the same benefits that you had while you were running. An easy way to have the workout go by quicker is to perform a fartlek where you may go 2 minutes hard and one minute easy, alternating back and forth. In the pool this may be 2 laps easy and one lap hard. Feel free to change up the intervals to keep things fresh!
Usually I would not recommend for a healthy runner, who is attaining peak mileage, to add cross training into their regiment. It will only lead to over training and unnecessarily fatiguing your legs, causing your running workouts to suffer. The exception is if you are an injury prone runner who is unable to run much mileage for fear of injury. If this is you then adding in a few cross training sessions a week is definitely a good idea. The approach here however is different than if you are not running at all. You want to experience aerobic benefits without making your legs tired for your running so it is best to choose the activities that are most different from running. Thus lap swimming is your best option and elliptical-ing is down the list. I also recommend only performing sessions of an hour or less on your cross training days as you likely already performed a good amount of running that day.
Its easy to get caught up in the cross training cycle as runners are always looking for that edge to make them a little more fit and a little stronger. Cross training needs to be approached with caution however as it may end up hurting you in the long run through injury or fatigue. If used correctly though you stand to make great strides from these activities!