Purpose: To evaluate the within-athlete sensitivity, validity, and dose–response relationships of exercise heart rate (HRex) from a submaximal fitness test (SMFT) as a proxy measure of cardiorespiratory fitness. Methods: This study follows an observational, repeated-measures design. Twenty-five trained female football players’ training loads (GPS and HR metrics) were collected throughout an 8-week preseason period. A 4-minute continuous-fixed SMFT protocol was administered weekly to evaluate HRex. A running time-trial assessment was conducted in weeks 2 and 8 to calculate mean velocity (5-min–30-s time-trial mean velocity) as a proxy measure of cardiorespiratory fitness. SMFT HRex measurement properties were determined via within- and between-athletes correlations and linear mixed models. Results: The overall preseason change in SMFT HRex derived from weekly repeated measures was −4.7% points (90% CIs, −3.9 to −5.6). The association between SMFT HRex and 5-minute–30-second time-trial mean velocity changes was large (−0.55), with 90% CIs ranging from negative moderate to negative very large magnitudes (−0.31 to −0.71). A 1% point decrease in SMFT HRex corresponded to an increase in 5-minute–30-second time-trial mean velocity of 0.13 (90% CIs, 0.03–0.24) km·h−1. Within-athlete correlations between training loads and SMFT HRex demonstrated moderate inverse relationships with external-load parameters (−0.39 to −0.47) and small inverse relationships with HR-based internal metrics (−0.25 to −0.29). Accumulated high- and very high-speed running throughout the entire preseason showed consistent inverse dose–response relationships with SMFT HRex (moderate and large, respectively), while all other load measures displayed unclear associations. Conclusions: SMFT HRex is a sensitive and valid tool to evaluate preseason cardiorespiratory-fitness changes in female football players.