Too much sitting is taking years off your life

That seems to be how this study is being reported anyway.

Sedentary behaviour and life expectancy in the USA: a cause-deleted life table analysis (full paper)

Interesting to see all the commentary on this one in various places:

The BBC - Can spending less time sitting down add years to life?

Science Daily - Cutting Daily Sitting Time to Under Three Hours Might Extend Life by Two Years

Obesity Panacea made a good point:

These sorts of theoretical studies obviously need to be taken with a large dollop of salt (just like the recent Australian study which estimated that every hour of TV viewing shortens your life by 25 minutes).  The point is simply that there is a non-negligible impact of sitting/TV viewing on mortality, and given the extremely high prevalence of these behaviours at the population level, they can have noticeable impact on the lifespan of the population as a whole.

As usual, NHS Choices keep things on an even keel, nice and calm....

ConclusionIn conclusion, the research does not alter the current advice in the UK for adults to have 150 minutes of moderately intensive activity such as cycling or fast walking each week. This research provides some evidence to put numbers on the link between sedentary behaviours such as sitting or watching television with life expectancy. It’s important to note that this type of research cannot determine that one causes the other; only what the size of an effect might be when given the link. Although the researchers corrected for confounders such as age and gender, nothing else was adjusted for.
Furthermore, there may have been other factors at play, such as smoking or illness, which influenced the results. There are other limitations to this study, some of which the authors noted:

• This study was based on the results of US participants, and therefore its findings may not apply to the UK population.

• Sedentary behaviours were identified by self-report, which makes the results less reliable. It is possible that participants did not accurately report their activity patterns and people may have felt more inclined to describe themselves as more active than they actually were.

• Life expectancy as noted by the authors is a population statistic and does not apply to individuals. The authors noted that these estimates are theoretical estimates only. 

Consequently, the Mail’s headline “Stand up if you want to live longer,” is misleading to the reader and should be interpreted with caution, although most of us could benefit from being more active.