Researchers say current exercise guidelines are unrealistic and argue that doctors should sometimes advise small increases in activity instead.
They warn the 150-minute weekly target is beyond the reach of some people - particularly older individuals.
And striving to reach these goals could mean the benefits of lighter exercise are overlooked.
But public health officials say current recommendations have proven benefits in lowering the risk of heart disease.
There is mounting evidence that inactivity is linked to heart disease, diabetes and some types of cancer.
UK guidelines for adults recommend at least two-and-a-half hours of moderate activity a week, in bouts of 10 minutes or more.
But in two separate articles in the BMJ, experts argue the message needs to change, with greater emphasis on making inactive people move more.
Prof Philipe de Souto Barreto at the University Hospital of Toulouse, advises people who are sedentary to make small incremental increases in their activity levels - rather than pushing to achieve current goals.
He points to previous studies which show even short periods of walking or just 20 minutes of vigorous activity a few times a month, can reduce the risk of death, compared to people who do no exercise.
In the second article, Prof Phillip Sparling of the Georgia Institute of Technology, says doctors should tailor their advice - particularly for older patients.
He suggests using GP visits for people over 60 to discuss "realistic options" to increase activity - such as getting people to stand up and move during TV commercial breaks.
Prof Kevin Fenton at Public Health England, says: "Everyone needs to be active every day - bouts of 10 or more minutes of physical activity have proven health benefits, but getting 150 minutes or more of moderate activity every week is the amount we need to positively impact on a wide range of health conditions.
"This includes reducing the risk of heart disease and type 2 diabetes."