Research by the British Heart Foundation (BHF) found some 48 per cent of women and 42 per cent of men confessed to feeling so unfit they would get short of breath if they had to run to catch public transport.
The survey also found 47 per cent felt they could run up to just half a mile, while one in five said they could only manage 100 metres.
Even of those who believed they could run a mile, three in 10 (29 per cent) said they had last done so more than a decade ago, the survey of 2,000 adults by One Poll showed.
Almost two-fifths (38 per cent) said they did not feel fit enough to join a gym and 28 per cent thought running was for those who are already fit and healthy.
The BHF called the figures “worrying”, warning heart and circulatory disease affects around seven million people in the UK and is responsible for around 155,000 deaths each year - an average of one person every three minutes.
The charity has launched a new fundraising campaign called MyMarathon.