Go for the burn. One advantage of strength training is that you can target specific muscle groups and prevent injuries by concentrating on strength imbalances. Or strengthen areas of the body where you had a previous injury. Pick a weight that you can lift for ten to 15 repetitions. The last one or two reps should be difficult, maybe even impossible to finish.
Indoor machines offer a good substitute at a relatively low cost.
The straight-forward movements of ski simulators come close to matching those used in running, but with less impact. Or if you decide to simply swim, you'll help strengthen your upper body while relaxing your other muscles. Water training works particularly well if you have an injury and can't run, but it's also good as a cross-training discipline for preventing future injuries. Indoor bikes are particularly good for providing active recovery between hard running workouts. Next to the treadmill, you'll get a more specific running workout on a stair climber than on any other machine.