The importance of  both LSD and HIT training for improved athletic performance are demonstrated in the research. Baumann et al. (2011) have conducted a neat study highlighting the importance of both aerobic and anaerobic fitness on 5km time-trial performance.

A group of trained cross-country runners (n=13) were tested for repeated sprint ability (RSA 50m), maximal anaerobic speed, for VO2MAX and VMAX, for running economy and ventilatory threshold. They were also tested for their 5km time-trial performance. The study took multiple measures in order to see what factor or combination of factors contributed to performance variance. The results demonstrate that VO2MAX, maximum anaerobic speed and ventilatory threshold accounted for 81% of performance variance, with maximum anaerobic speed accounting for 31% alone.

This is another confirmation that VO2MAX alone is not a predictor of time-trial or race success. We already know this from marathon runners, where runners with identical aerobic power values clock significantly different times. A 5km race is highly aerobic, and this study reveals the importance of both the aerobic and anaerobic systems.