Building on our previous findings that once-weekly high-intensity exercise, whether performed as a single bout or as three-set intervals, effectively improves cardiorespiratory function, this study investigated the effects of even lower frequency―biweekly―high-intensity interval training (HIIT) on cardiorespiratory function in healthy adults, focusing on maximal oxygen uptake (V_O2max) and ventilatory threshold (VT) as primary indicators of aerobic capacity. We hypothesized that a reduced-frequency HIIT protocol would significantly improve these cardiorespiratory function indicators over a 4-month period. Eighteen healthy adult volunteers (age: 20.7 ± 0.9 years) participated in the study. The HIIT program consisted of 8 sessions over 4-months, conducted once every two weeks. Each session included three sets of cycling at 80-90% of the maximum work rate (WR_max) until exhaustion. Cardiorespiratory function was assessed through ramp exercise tests before and after the intervention. After the 4-month HIIT program, the training group showed significant improvements in several key indicators of aerobic fitness. V_O2max, the gold standard measure of cardiorespiratory fitness, increased by +12.7 ± 10.2% (p = 0.005). WR_max during the ramp exercise test increased by 10.4 ± 5.4% (p = 0.001). VT, an important submaximal indicator of endurance capacity, also improved significantly (+14.4 ± 16.1%, p = 0.021). Progressive improvement in exercise performance was observed throughout the training period, with maximal exercise duration showing significant increases: month 1 (188 ± 69 s), month 2 (207 ± 72 s), month 3 (250 ± 102 s), and month 4 (295 ± 133 s), representing a 57.2% total improvement (ANOVA p = 0.002). Individual comparisons of pre- and post-HIIT V_O2max and VT in the training group consistently showed improvements. The control group showed no significant changes in these parameters. This study provides evidence that biweekly low-frequency HIIT can induce significant improvements in both maximal and submaximal cardiorespiratory function in healthy adults. The results suggest that substantial cardiorespiratory benefits can be achieved with much less time commitment than traditionally recommended, potentially increasing exercise adherence in time-constrained populations. Further research is needed to explore the long-term effects, underlying mechanisms, and applicability to diverse populations.