The aim of this study was to determine whether basic fitness parameters have the impact on the specific military activity such as walking 18 km with 25 kg of load. The members of Croatian Armed Forces (30 soldiers) were tested before the beginning of the training program. The study has included variables for the assessment of muscular endurance: push-ups in 2 minutes, sit-ups in 2 minutes, maximum number of pull-ups before dropping from the bar, bench press with 70% of body weight-max number of repetitions, max number of squats for 60 seconds, then the variables for the assessment of aerobic capacity: the 3200m run and relative oxygen uptake using the direct method of measurement on a treadmill as well as the variable for the assessment of body fat (body fat %). As the criterion variable, it was used the 18 km walking with 25 kg of load. The results of the regression analysis have shown statistically significant relation of predictor variables with the criterion variable. The two variables, 3200m run and RVO2 had a significant Beta coefficient. Based on the obtained results it could be concluded that great cardio-respiratory endurance has a much larger impact on the walking length of 18 km with a load of 25 kg than other fitness parameters.