ABSTRACT Introduction Cardiorespiratory fitness (CRF) is rarely measured in population studies. Most studies of CRF do not examine differences by population subgroups or seasonal trends. We examined how estimated CRF levels vary by anthropometric, sociodemographic, and behavioral characteristics in a population-based cohort of UK adults (the Fenland Study). Methods We used a validated submaximal exercise test to obtain CRF estimates (CRF_estimated) in 5976 women and 5316 men, residing in the East of England. CRF_estimated was defined as estimated maximal oxygen consumption per kilogram total body mass (V̇O₂max_tbm) and fat-free mass (V̇O₂max_ffm). Descriptive statistics were computed across anthropometric and sociodemographic characteristics, and across the year. Progressive multivariable analyses were performed to examine associations with physical activity energy expenditure (PAEE) and body mass index (BMI). Results Mean ± SD V̇O₂max_tbm was lower in women (35.2 ± 7.5 mL·min⁻¹·kg⁻¹) than men (41.7 ± 7.3 mL·min⁻¹·kg⁻¹) but V̇O₂max_ffm was similar (women: 59.2 ± 11.6 mL·min⁻¹·kg⁻¹; men: 62.0 ± 10.3 mL·min⁻¹·kg⁻¹). CRF_estimated was inversely associated with age but not after adjustment for PAEE. People in more physically demanding jobs were fitter compared with those in sedentary jobs, but this association was attenuated in women and reversed in men after adjustment for total PAEE. Physical activity energy expenditure and BMI were positively associated with CRF_estimated at all levels of adjustment when expressed relative to fat-free mass. CRF_estimated was 4% higher in summer than in winter among women, but did not differ by season among men. Conclusions CRF_estimated was inversely associated with age but less steeply than anticipated, suggesting older generations are comparatively fitter than younger generations. Physical activity energy expenditure and BMI were stronger determinants of the variance in CRF_estimated than other characteristic including age. This emphasizes the importance of modifiable physical activity behaviors in public health interventions.