Background: The knowledge, from laboratory studies dating back to the 1950s on the importance of the association between cardiorespiratory fitness and aerobic workload for workers health, is fundamental for promoting sustainable healthy employability among ageing blue-collar workers today. However, the association between cardiorespiratory fitness and aerobic workload has not yet been documented during daily work, and we do not know if it applies to the normal work of blue-collar workers in different age groups. We aim to investigate the association between cardiorespiratory fitness and aerobic workload among blue-collar workers using measurements of 24-h heart rate collected over consecutive working days. Methods: We analyzed baseline cardiorespiratory fitness, assessed using a sub-maximal cycle ergometer test, and 1–4 days of 24-h heart rate measurement from 497 blue-collar workers participating in the DPHACTO study. We investigated the association between cardiorespiratory fitness and aerobic workload defined as the average percentage of heart rate reserve (%HRR), maximum %HRR and the duration time spent at a high HRR (> 30%) during working hours. The association was assessed using multivariate linear regression models adjusted for age, sex, self-rated health, shift-work, prescription medication and occupation, as well as for different age strata. Results: Higher cardiorespiratory fitness was significantly associated with decreased mean %HRR −0.32 [95% CI −0.39 to −0.25], maximum %HRR −0.35 [95% CI −0.45 to −0.25] and time spent at ≥ 30% HRR; −1.8% [95% CI −2.2 to −1.5%]. These associations were evident across age groups, with slightly stronger associations for workers aged 46–51 (total range 18–68). Conclusions: Higher cardiorespiratory fitness was associated with the decreased aerobic workload during normal work across all age groups and levels of work intensity. Our findings highlight the importance of cardiorespiratory fitness when considering the workload and its relevance in the promotion of healthy sustainable employment.