Thank you to all of you who commented and shared your thoughts on this topic. After reading your comments I did a little more research online and came to the conclusion that it is not ultra-endurance sporting activities that cause excessive oxidative stress. Instead, it is overtraining for such activities and events that is the true culprit. I think ultra-endurance athletes have a reputation for overtraining. But as long as one doesn't overtrain, then the unhealthiness of oxidative stress can be avoided while being a randonneur it seems. Without proper recovery periods following workouts something unhealthy is bound to occur.