Most of the distance racers I have known (including myself) will simply keep increasing training intensity and/ or duration until they suffer an injury that interrupts that. Proper equipment, running mechanics, and training practice all help increase one's injury threshold, but the potential for injury never disappears. To race to one's potential generally involves learning how to suppress physical distress signals, and some of these can indicate an incipient injury. It took me a few years to distinguish ordinary racing/training distress from injury-related distress. It's difficult to make an enthusiast back off.