There have been inquiries on this list in the past about negative health impacts of endurance sports. there's a new article that just came out in the June 2012 issue:
O'Keefe et al 2012. Potential Adverse Cardiovascular Effects from Excessive endurance exercise. Mayo Clin Proc 87(6) 587-595.
The authors suggest that chronic trining for and competeing in extreme endurance events (including 'very' long distance bicycle races) may be associated w/ transient reduction in right ventricular ejection fraction (pumping capacity of the heart), and elevation of cardiac biomarkers that indicate myocardial injury. The changes are transient, lasting 1 wk to 1 month in young healthy males. With long term repetitive injury, this process can lead to myocardial fibrosis leading to a higher risk of heart arrhythmnias, coronary artery calcification, diatolic dysfunction and increased vascular stiffness. Veteran endurance atheletes have a 5-fold increase in the prevalence of atrial fibrillation (which can lead to sudden death). Increased vascular stiffness is associated w/ hypertension.
The authors acknowledge that the process is not well understood and why some individuals seem to be susceptible to problems while others are not is not known (So I guess the warnings are 'hypothetical' rather than evidence based). The article reviews data from events that are by definition 'extreme' (competitive marathons). Who knows what happens on a long and slow ride.
Remember that people who exercise regularly have lower rates of disability and a mean life expectancy that is 7 yrs longer than inactive people. Routine exercise (e.g., running) is associated with a 19% lower risk of all-cause mortality compared w/ sedetary folks.
On a side note... we study bone metabolism in our group and have done studies on competative cyclists (Barry et al 2011 Acute calcium ingestion attenuates exercise-induced disruption of calcium homeostasis Med Sco Ports Exerc 43(4):617-623). In a recent sample of twenty-ish yr old male high-end racers- about a third had osteopenia/osteoporosis (thinning of the bones). The loss of bone calcium over the season is pretty impressive. As above, the effect is assumed to be related to the intensity of exercise. It's unknow what might be going on with Brevet riders. Under the advice of your doctor, consuming calcium during or after a ride couldn't hurt.
Be careful out there and take good care of yourself!