Even with the COVID-19 pandemic time is usually a premium for training and if this is true for your situation then performing compound movements is the quickest method to gain benefits for endurance sports [running or biking ultras or both—think Leadman preparation, triathlon, adventure racing, etc.].
Classic compound movements include the bench press, squats, overhead press, and dead lift, but could include the clean and jerk and other Olympic lifts. The Olympic lifts require technique training to perform safely and to get the most training benefit.