Too much of anything is as bad as too little of anything. Some people are built (genetically) to run very long distance, others, not so much. There is evidence of cardiac scar tissue in these individuals that run long distances for long periods of time. Year after year after year. What will be interesting is to find out for each individual what is considered "long distance". Everything we do is cumulative and we pay a physiological price for it. We will eventually find the optimal zone for every individual, and they will likely be different for every individual. It's all in our genes.