It’s common advice for runners to get in some cross-training: “improved [] fitness, injury prevention and rehabilitation, quicker recovery, and boredom busters.” My preferred cross-training activity is stationary rowing. It’s a great full body workout, it’s non-impact, and the calorie burn/effort level/pacing is pretty close to equivalent for running (i.e., rowing 10K is pretty close to running 10K in most respects).