Back in the day, did competitive distance running. Certain types of runs we did in the "time-trial" format. Tough, each of them. As suggested, it can be as much (or more) of a mental challenge as physical. Each choice made can mean seconds or minutes of difference. Sometimes, particularly on tougher, hill, technical courses, one wrong step could mean the difference between injury or gaining on the clock. Hugely satisfying, though, when getting it "right" and handling the mental aspect. Generally, those for me were the runs when the physical simply took care of itself.