Here's a good study:
http://www.onlinejacc.org/content/65/5/411
This study suggests that "slow", "short", and <3x per week is optimal, but cites an upper limit on healthy running from an older study:
"A weekly energy expenditure of 3,500 kcal is approximately equivalent to that required for running 35 miles, which is in the range of the upper limits for incremental health benefits from strenuous exercise identified by several recent large epidemiological reports. These studies found that a weekly cumulative dose of approximately 30 miles of running or 46 miles of walking is approximately the safe upper limit for optimizing long-term CV health and life expectancy."
"100% of the people who confuse correlation and causation end up dying."
http://www.onlinejacc.org/content/65/5/411
This study suggests that "slow", "short", and <3x per week is optimal, but cites an upper limit on healthy running from an older study:
"A weekly energy expenditure of 3,500 kcal is approximately equivalent to that required for running 35 miles, which is in the range of the upper limits for incremental health benefits from strenuous exercise identified by several recent large epidemiological reports. These studies found that a weekly cumulative dose of approximately 30 miles of running or 46 miles of walking is approximately the safe upper limit for optimizing long-term CV health and life expectancy."
"100% of the people who confuse correlation and causation end up dying."