The problem is that it takes time; for the same amount of time you can have a better &quot;training effect&quot;.  For less time you can have the same training effect.  An hour sauntering around the city takes as much time as an hour biking at your lactate threshold.  But your heart works a lot harder for that hour of biking compared to standing around waiting for traffic lights to change.<p>Both are better than an hour spent replying to comments on HN, though!