It's come to my attention that people don't distinguish between running and training. Once we clear up the differences, we can figure out how to resume training after recovery from an injury.
If you're a runner, you get up, put on your shoes, and go for a run a few times a week. Sometimes a bit longer. Sometimes a bit shorter. If you're training, however, you might be following a schedule, you probably run a couple of workouts every week, and you're generally trying to go faster, farther, or both often with a target race in mind.
When you return to running from an injury, you don't always need to do a whole lot (except the exercises your physical therapist shows you). You just build back up, slowly, to running a couple of days a week, and, boom, mission accomplished.
To return to training, however, you first must return to running (see above) but then you have to bridge that gap. (You can also train through an injury while recovery and seeking treatment but you need a highly knowledgeable coach or athletic healthcare professional to help guide your through that).
If you're returning to training after an injury, then you need to do way more than just build up slowly until you're back to where you were. You also need to train! How do you do this? By building your volume (time performing cardiovascular/endurance activities) through cross-training. But not cross-training like most people think of it.
To most people, cross-training is basically anything other than running. But cross-training in this case is best defined as the following: exercise that closely mimics your primary sport (running). (You also need to be doing the other "cross-training" things like strength training (body weight or more), TRX, Pilates, etc.).
Your goal is to build a cross-training routine that mimics your running training as much as possible. If you don't keep up your normal volume of cardiovascular/endurance exercise, then the things that make you healthy and fit will start to deteriorate. Your hormonal system will not be primed to respond to your normal running volume or workouts, which will make your return way harder. Your cardiovascular system will be undertrained, slowing you down and making your training feel pretty crappy. And your bones will lose strength and density while your muscles atrophy from less use. Doesn't that sound terrible? This is why cross-training is your key to success when you're returning to training after an injury. But remember, not all cross-training is created equal.
Hopping on the elliptical, stationary bike, or in the pool for aqua jogging (or an Alter-G for the lucky, lucky few!) are going to be the best ways to improve or maintain your cardiovascular, musculoskeletal, and hormonal systems so you stay fit and in shape while building your training back up.
Cross-training should mimic your normal running schedule as much as possible. If you would normally do an easy run, then don't take a spin class instead! Just do the equivalent of an easy run on a stationary bike. If you were going to do an interval workout at the track, then convert the distances that were supposed to be run into minutes and do the intervals on a bike, elliptical, or in the pool aqua jogging.
Let's look at an example. If your used to running 5.5 hours per week (35 miles per week at an average pace of 10 minutes per mile), and you're coming back from an injury, then maybe your exercise schedule might look like the following:
Monday: 1mi warm up, 3mi tempo run, 1mi cool down = 50 minutes
Tuesday: 45 minutes EASY on the elliptical, stationary bike (not spin class), or aqua jogging (not swimming)
Wednesday: Strength training with weights (don't forget to strengthen your legs with squats, jumps, etc.) = 30 minutes
Thursday (speed workout): 1 mile warm up, 6 x Buck hill repeats, 1 mile cool down
Friday: 45 minutes EASY on the elliptical, stationary bike (not spin class), or aqua jogging (not swimming)
Saturday (long run): 60 minutes on the elliptical, bike, or aqua jogging + 5 miles run
Sunday: 45 minutes easy on the elliptical, stationary bike (not spin class), or aqua jogging (not swimming)
If you can only do one workout per week of running, then you might sub out that tempo run for a tempo elliptical or aqua jog session. That session might look like this: 15 minutes easy, 30 minutes at tempo run effort, 15 minutes easy. Or if you have to sub out a speed workout for a cross-training session, then that session might look like this: 15 minutes at an easy effort, 8 x 2 minutes comfortably hard, 1 minute super easy for your recovery interval, 15 minutes at a very easy effort to cool down.
Does this mean you need a gym membership? Not necessarily. We have a stationary bike at home (basic ones can run as low as $169, which is basically the price of a couple months of gym membership dues). But, you can also find some very inexpensive gym memberships if all you're looking for is an elliptical or stationary bike.
In summary: to return from an injury or to keep training while battling an injury, you need to keep up the same level of aerobic exercise you would be doing if or when you were training before you were hurt. So don't just limit your aerobic exercise to a few miles per week and some yoga IF you want to be training for a race. Keep up the same level of exercise by hitting the gym (or home gym) and staying fit. Here is the rank of the best cross-training exercises in order of effectiveness to mimic run training:
Alter-G
Aqua jogging (google it)
Elliptical
Stationary bike
Special thanks to Katherine Voepel Ward for adding her observations and expert opinion to this post