You're a fitter man than I, oldspark! I don't "push" myself deliberately - on days when I feel strong, I ride harder. On days when I'm ho-hum, I take it easy. In January, when I started, all my rides were difficult, to some extent. Now, being somewhat fitter, I have more days that I feel like riding harder. As the weather has warmed, I've been slowed, a little. The 95-degree plus heat makes it harder to feel like really pushing because there's no good way to keep the sweat out of my eyes except to stop, take off my glasses, and wipe with the sleeve. I don't mind the heat, but I do feel tired sooner. Since my typical ride is about 12 or so miles, I bring a water bottle with me to consume halfway. I've noticed no immune system problems, but I suspect I don't push myself nearly as hard as you do.