>"According to several doctor friends of mine, balance is the best way to go for a long life."

From what I understand, this is just the "moderation is good, extremes are bad" meme. Similar things are recommended regarding diet composition etc. I usually read it as "we don't really know, so just do what everyone does".

>"And of course, exercise only helps up to a normal level."

What's normal? You think people only spent 10 minutes a day running, in the past?

1 hour is 10 km at a leisurely pace. If I go for a run in the morning, and in the evening, I'm very confident that I'll have better chances to live longer than someone who runs less than 2 km a day. Running ultramarathons will shorten your life, maybe. But not a jog or two.

---editing to add details after googling around a bit---

Intensity of physical exercise is measured in metabolic equivalents (MET)[1]. There is also a table with MET values for common activities.

I looked at some studies, but most are not interesting. One[2] defines "vigorous PA" like this:

Assessments of vigorous physical activity were based on a single set of intensity categories applied at all three time points; these were: 1) walking, 2) alternately walking and jogging, 3) jogging (light running), and 4) running. Vigorous activity was defined as levels 2, 3, or 4 (all more intensive than normal walking), performed at least 3–5 times/month in 1975 and 1981, or at least for 30 min each week in 1990.

(so - not useful)

However, I found another one[3], which has this to say:

Several of us have recently analyzed 55,000 individuals (13,000 runners) from the ACLS database during nearly 15-year follow-up to assess the impact of running on CVD and allcause mortality.11 Compared with nonrunners, runners had a reduction in all-cause and CVD mortality of 30% and 45%, respectively, with an average increase in survival of 3.0 and 4.1 years for all-cause and CVD-related survival, respectively, after adjusting for lifestyle factors (eg, smoking and obesity) and medical conditions (eg, HTN and T2DM). Persistent runners appeared to receive the full benefit from mortality reduction, whereas those who started running but stopped or those who were not running at baseline but subsequently started running appeared to receive nearly half of the benefit received from CVD and all-cause mortality reduction. These results are impressive, though perhaps not surprising when considered in the context of the data outlined in previous sections, showing myriad health benefits of running.

However, when assessing doses of running, somewhat surprising results were found. Runners generally had considerably higher levels of CRF than did nonrunners, and moreover, CRF levels in runners progressively increased with increasing doses of running (Figure 2).11 Most evidence indicates better survival with estimated MET levels greater than 10,23-25 although some evidence indicates progressively better survival with even higher CRF levels.45 When 13,000 runners were divided into quintiles of running doses (miles per week, times per week, minutes per week, and speed), no significant differences in the benefits were noted for any of the running groups.Infact,runnersinquintile1(eg,<6 miles/wk, 1-2 times/wk, <51 min/wk, <6 mph) received the full benefits from running with regard to reduction in CVD and all-cause mortality (Figure 3).11 These results suggest that maximal benefits of running occur at quite modest jogging doses and that runners in quintile 1 have almost similar benefits of running compared with those in quintile 2 to quintile 4 and have a slight, nonstatistically significant trend toward greater benefitcomparedwith those in quintile 5. However, in contrast to the results of the Copenhagen City Heart Study,14 our results, from a larger sample with much better statistical power, indicated that runners with high doses of running in quintile 5 still had significantly better CVD and all-cause survival compared with nonrunners.11 However, these higher doses of running were not necessary to achieve maximal reduction in CVD and allcause mortality.11-14

1 - http://onlinelibrary.wiley.com/doi/10.1002/clc.4960130809/pd...

2 - http://www.nature.com/articles/srep18259

3 - http://www.sciencedirect.com/science/article/pii/S0025619615...