Purpose: To assess the reliability and construct validity of a self-paced, submaximal run test (SRT$$_{RPE}$$) for monitoring aerobic fitness. The SRT$$_{RPE}$$ monitors running velocity (v), heart rate (HR$$_{ex}$$), and blood lactate concentration (B[La]), during three 3-minute stages prescribed by ratings of perceived exertion (RPEs) of 10, 13, and 17. Methods: Forty (14 female) trained endurance runners completed a treadmill graded exercise test for the determination of maximal oxygen consumption (VO$$_2$$max), v at VO$$_2$$max (vVO$$_2$$max), and v at 2 mmol·L$$^{−1}$$ (vLT1) and 4 mmol·L$$^{−1}$$ (vLT2) B[La]. Within 7 days, participants completed the SRT$$_{RPE}$$. Convergent validity between the SRT$$_{RPE}$$ and graded exercise test parameters was assessed through linear regression. Eleven participants completed a further 2 trials of the SRT$$_{RPE}$$ within a 72-hour period to quantify test–retest reliability. Results: There were large correlations between v at all stages of the SRT$$_{RPE}$$ and VO$$_2$$max (r range = .57–.63), vVO$$_2$$max (.50–.66), and vLT2 (.51–.62), with vRPE 17 displaying the strongest associations (r > .60). Intraclass correlation coefficients (ICC$$_{3,1}$$) were moderate to high for parameters v (range = .76–.84), HR$$_{ex}$$ (.72–.92), and %HR$$_{max}$$ (.64–.89) at all stages of the SRT$$_{RPE}$$. The corresponding coefficients of variation were 2.5% to 5.6%. All parameters monitored at intensity RPE 17 displayed the greatest reliability. Conclusions: The SRT$$_{RPE}$$ was shown to be a valid and reliable test for monitoring parameters associated with aerobic fitness, displaying the potential of this submaximal, time-efficient test to monitor responses to endurance training.