A poll for the walking organisation the Ramblers suggests 25% of adults walk for no more than one hour each week.
The YouGov survey, which polled 2,000 adults to find out where and when they walked, found another 43% reported walking for less than two hours a week.
Government guidelines say people should do 150 minutes of "moderate" physical activity each week.
The survey asked people about the total amount of walking they did, including trips to school, work or the shops.
The Ramblers, who are promoting a "Get Walking Week" from 4-11 May, say there is already research showing that two thirds of adults in the UK do too little exercise - and that this survey backs that up.
'Inactivity pandemic'Despite the low levels of walking reported, virtually all those surveyed - 93% - agreed walking was a good form of exercise.
The organisation is seeking to get people to go on walks of five miles or less and has free walks led by guides.
Benedict Southworth, chief executive of the Ramblers, said: "Walking is one of the most accessible and achievable ways to truly conquer this inactivity pandemic in Britain, and we need to get started now."
Public health minister Anna Soubry, said walking was "one of the best ways to keep healthy", and that she supported the initiative.
/BBC/