High-intensity exercise acutely elevates blood glucose levels, yet the underlying links between this acute glycemic surge, cardiorespiratory fitness, and cardiac autonomic regulation remain unclear. We investigated whether an acute post-exercise glucose increase (ΔCGM >2 mmol/L) during cardiopulmonary exercise testing (CPET) reflects specific fitness, autonomic, and glycemic characteristics. Using continuous glucose monitoring and single-lead ECG data from 47 healthy active adults (BUT CGM dataset), participants were categorized into a glucose increase (n = 20) and a non-increase group (n = 27) based on the glycaemia response to GET (ΔCGM). Linear mixed-effects models were applied to evaluate multi-day glycemic variability and multi-night nocturnal heart rate variability (HRV). The glucose increase group demonstrated significantly higher VO2max, greater pre-exercise SDNN and total power, lower nocturnal HR, and elevated nocturnal pNN50, lnRMSSD, and total power. Glycemic variability across four days was also higher in the increase group. Crucially, no group × time interactions were significant for nocturnal HRV or glycemic variability, indicating that these differences were stable traits present prior to exercise rather than transient post-test perturbations. In conclusion, the acute glucose excursion during maximal exercise identifies a stable, individual-level metabolic–autonomic phenotype rather than a simple acute stress response.