Do your strength training first and then go running. Or is it the other way around? Does it matter in which order you do them, or is it irrelevant in terms of your training success? As is so often the case, there is no simple yes or no answer. It all depends on your training goal.
You want to build muscle and improve your maximum strength
If your goal is to build muscle and/or increase your maximum strength, then you should definitely do your strength session before your endurance training. You should never attempt an effective strength building workout when your muscles are already fatigued. You can’t work out at the intensity necessary to provide an ideal training stimulus. Since sometimes you will be lifting very heavy weights in your muscle and maximum strength training, fatigued muscles increase the risk of injury. If you tire your muscles out ahead of time, your coordination will suffer and your stabilizing muscles will be weakened.
After your strength session, it’s probably not a good idea to add on an endurance run. This can interfere with the muscle building process. Your body needs plenty of recoveries to repair and rebuild muscle tissue.
You want to improve your running endurance
If your goal is to improve your endurance, then go for your run before your strength training. To produce an effective training stimulus, your muscles should be rested before long and/or intensive running sessions. Tired muscles prior to running negatively impact your economy of movement and your running form. This can then lead to strain and overuse injuries in your joints and muscles.
When you feel like you simply must go running after a strength workout, then make sure to keep your run short and the intensity low in the basic endurance zone.
You want to improve your overall fitness
In this case, you can basically do your cardio and strength training in whichever order you want. But you should still define a specific training goal for each session. That way you can get the most out of your workout.
You want to lose weight
It is often recommended to do your strength training before your endurance run to empty your carbohydrate stores. The idea is to force your body to get its energy primarily from fat rather than carbs during your run. However, the problem with this strategy is that you can’t finish a long distance run at high intensity on empty carbohydrate stores. While it is true that a much higher percentage of fat is burned for energy, the calorie burn, on the other hand, is relatively low because of the low intensity of the workout. If you are looking to lose weight, a negative energy balance is key: If you burn more calories than you consume, you will lose weight. In the end, what matters is how many calories you burn in total through your workout. Spread your workouts out over several days. That way you can train at high intensity and burn a lot of calories, and at the same time give your body the time it needs to recover properly before the next workout.
In general, you should not do two workouts back-to-back. You will achieve better results in both your strength and endurance training if you give your body sufficient time to recover. If you nevertheless want to combine your strength and endurance training, then you should follow the order best suited for your specific training goal.