Walking is a safe, accessible and low cost activity, amenable to change and known to have great potential to increase physical activity levels in sedentary individuals. The objective of this study is to estimate the proportion of the 2009 adult population of England who would attain or exceed vigorous intensity activity (>70% maximum heart rate [HRmax]) by walking at 3 mph. We conducted predictive impact modelling using participants' (n = 1741, aged 25–64 years) cardiovascular fitness data from treadmill walking tests. We combined this data with English population estimates adjusted for age and sex to estimate the numbers of individuals that would exceed 70% HRmax (an intensity considered sufficient for fitness gains) when walking at 3 mph (4.8 km · h−1). We estimate 1.5 million men (95% confidence interval [CI] 0.9–2.2 million) (from 13.4 million corresponding to 11.6% (95% CI 7.0–16.2%)) and 3.9 million women (95% CI 3.0–4.8 million) (from 13.6 million corresponding to 28.6% (95% CI 22.0–35.1%)) in England aged 25–64 years would benefit from regularly walking at 3 mph. In total, a projected 5.4 million individuals (95% CI 3.9–6.9 million) aged 25–64 (from 26.97 million corresponding to 20.1% (95% CI 14.6–25.7%)) could benefit from walking at 3 mph. Our estimates suggest a considerable number of individuals in the English population could receive fitness and health benefits by walking regularly at 3 mph. Physical activity messages that promote walking at this speed may therefore have the potential to significantly impact national fitness levels and health in England.