Background  People who are physically active have at least a 30% lower risk of death during follow-up compared with those who are inactive. However, the ideal dose of exercise for improving longevity is uncertain.
Objectives  The aim of this study was to investigate the association between jogging and long-term, all-cause mortality by focusing specifically on the effects of pace, quantity, and frequency of jogging.
Methods  As part of the Copenhagen City Heart Study, 1,098 healthy joggers and 3,950 healthy nonjoggers have been prospectively followed up since 2001. Cox proportional hazards regression analysis was performed with age as the underlying time scale and delayed entry.
Results  Compared with sedentary nonjoggers, 1 to 2.4 h of jogging per week was associated with the lowest mortality (multivariable hazard ratio [HR]: 0.29; 95% confidence interval [CI]: 0.11 to 0.80). The optimal frequency of jogging was 2 to 3 times per week (HR: 0.32; 95% CI: 0.15 to 0.69) or ≤1 time per week (HR: 0.29; 95% CI: 0.12 to 0.72). The optimal pace was slow (HR: 0.51; 95% CI: 0.24 to 1.10) or average (HR: 0.38; 95% CI: 0.22 to 0.66). The joggers were divided into light, moderate, and strenuous joggers. The lowest HR for mortality was found in light joggers (HR: 0.22; 95% CI: 0.10 to 0.47), followed by moderate joggers (HR: 0.66; 95% CI: 0.32 to 1.38) and strenuous joggers (HR: 1.97; 95% CI: 0.48 to 8.14).
Conclusions  The findings suggest a U-shaped association between all-cause mortality and dose of jogging as calibrated by pace, quantity, and frequency of jogging. Light and moderate joggers have lower mortality than sedentary nonjoggers, whereas strenuous joggers have a mortality rate not statistically different from that of the sedentary group.
Read that conclusion again! And then, let me tell you that it is NOT actually borne out by the study. And most especially the headlines inspired by the study, such as The Telegraph's "Fast running is as deadly as sitting on the couch," The Chicago Tribune's "New study shows casual joggers outlast ironmen over long term," and Time's "When Exercise Does More Harm Than Good."

Thanks to Forbes for publishing this summary of the statistical analysis!
 Forbes has done a brilliant analysis of the actual results of the study and how it can be correctly interpreted. 

 Briefly, (1) it is just an observational study, (2) conducted on a group too small to do a valid statistical analysis, (3) the authors themselves wrote in a separate editorial article, "...we still need more data to truly determine ‘is more actually worse?’ regarding exercise dose and prognosis"; and (4) the authors themselves "are among the leading proponents of the 'less is more' exercise philosophy, so they would be inclined to support this study if the support were at all warranted."

The Forbes writer concludes, " Journalists and scientists have an obligation to fairly and accurately report the results of individual studies, and they have the further obligation to place those results in the context of what is already known in the field. By reporting the results of this one quite limited study with little or no critical perspective of its details or the larger context of the research, they have once again helped perpetuate the scientific illiteracy and innumeracy that is fast becoming one of the hallmarks of our time.."

....What I've been saying!