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 (VO₂max), v at VO₂max (vVO₂max), 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 VO₂max (r range = .57–.63), vVO₂max (.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.