This research seeks to determine the effect of a 5-month-long programme of moderate physical activity on the physical fitness (strength, suppleness and stamina) of premenopausal women. The study included premenopausal women who were separated into two groups: those under 35 (n = 10) and those over 35 (n = 10). For the purposes of the study an intervention programme of aerobics, step and toning classes was set up that lasted 5 months with a weekly frequency of 3 alternate days and a duration of 60 minutes for each session at an intensity of between 60-70% of maximum heart rate reserve. The results showed that at the end of the programme of physical activity younger women had increased their posterior trunk suppleness and grip strength in both hands, although they only significantly improved their anterior trunk suppleness (p < 0.05). The older women improved in all fitness parameters and like their younger counterparts only achieved significant changes in their anterior trunk suppleness (p < 0.05). In the comparison between the two age groups, the women over 35 attained a greater improvement with respect to the younger women (p < 0.05) in maximal oxygen consumption and left hand grip strength. The study findings show the potential benefits of the exercise programme used on suppleness in women as well as the relevance of the programme for adult women who lessen the loss of physical condition and cardiorespiratory fitness caused by age.