Are short runs, e.g. less than 20 minutes beneficial?

Yes, short runs are beneficial.

This is notably more so when beginning or returning to running. Or when preceded by a period of heavy training, or in preparation for a subsequent one.

From a health perspective, running as little as 5 minutes per day at slow speeds (10 min/mi / 6:15 min/km) is associated with markedly reduced risks of death from all causes and cardiovascular disease. And the benefits tail off quite dramatically and even begin to reverse as duration increases. Similar benefits are also realised with running only once per week.

Furthermore, for trained individuals, such short runs continue to promote cardiovascular benefits and neuromuscular reinforcement. Crucially, they also provide the necessary mechanical loading to strengthen your structural system (bones, tendons, and ligaments), which adapts significantly slower than your cardiovascular system. Consequently, short runs are a great way to instigate relative rest — aiding recovery and helping to avoid injury by conditioning your body's foundation — whilst still having a positive impact on your overall training.