Unrealistic target?

So the US Government now recommends that adults do between 60 and 90 minutes of moderate to vigorous exercise most days to sustain weight loss, and at least one hour to prevent weight gain. If that is truly what is necessary, no wonder everyone is fat. That’s more training than I’m doing with the aim of running a 90 minute half–marathon in March 2005. Then again, it makes sense, because although I’m getting fitter, I haven’t lost any weight. But realistically, if losing/not gaining weight via exercise really is THAT hard, is exercise the relevant factor to highlight in managing obesity?