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. Once you pick up the rhythm, you'll almost feel like you're running. Keep the cadence up and maintain an erect posture. Rowing machines exercise the big muscles of both the upper and lower body, but the prime benefits are cardiovascular. Though less specific for developing running muscles, you can get your heart rate up on a rowing machine while avoiding impact.