Theres plenty of ways in which more exercise or more intensive exercise can be bad for you – but it depends on what your timeframe is. Mortality rates will be better for people who exercise moderately throughout their life than for people who are inactive. Intensive exercise can result in wear, tear and injury which in time can curtail your ability to exercise at all. So the net result of that exercise, if you look at the whole of life, is mortality rates that are no better, or worse than not exercising at all. Bad knees – bad news in later life.
With the study linked to though and a 12 year time frame – unless the sample set were all very old or very ill then hardly any of the people in the study should have died. The numbers in either part of the trial (moderate or intensive exercisers) who died must surely have been insignificant. The article doesn’t say how they died either – its obviously a study thats been cherry picked for a BHF press release – were encouraged to assume that people would have died of heart disease and similar chronic conditions. Maybe faster runners are more likely to be hit by a bus.