Being active is known to be beneficial but US researchers, writing in the journal Cancer, said they had found many women did too little.
Only a third met recommended activity levels.
UK breast cancer groups said women here also needed more support to keep active after having the disease.
The American study looked at the pre and post-diagnosis exercise levels of 1,735 women aged 20-74 who had breast cancer between 2008 and 2011 in North Carolina.
In the US and the UK, adults are recommended to do at least 150 minutes of moderate-intensity physical activity or 75 minutes of vigorous-intensity physical activity each week.
But this study found only 35% of women who had experienced breast cancer met the physical activity guidelines.
In the UK, campaigners said women here also needed to exercise more.
Caroline Dalton, of Breakthrough Breast Cancer, said: "Physical activity after a breast cancer diagnosis has been shown to improve a patient's chances of survival and there is also some evidence that it may help to reduce the risk of breast cancer returning.
"Keeping active may also help patients cope, both during and after treatment, by improving general health and wellbeing."
She added: "Although this study was conducted in America rather than the UK, the results suggest that women who have received a breast cancer diagnosis need better support to keep active."
"There are no specific guidelines in place at the moment to tell us precisely how much physical activity is needed after a breast cancer diagnosis, but Breakthrough Breast Cancer suggests aiming for 3.5 hours per week, after checking with your treatment team to see what is appropriate for you."
Baroness Delyth Morgan, chief executive at Breast Cancer Campaign, said: "This study serves as a reminder of how important it is that women with breast cancer are made aware that physical activity can improve their chances of survival.
"Recent research has shown that even small increases to the amount of exercise done after a breast cancer diagnosis can give women a better chance of survival.
"This is why it is essential that women are given a clear written follow-up care plan, which should include practical advice about diet and exercise."
Exercise can aid recovery after breast cancer but many women are not active enough, a study suggests.