Why is interval training so important? Specifically Threshold and VO2max intervals. Well, that's because these intervals usually replicate (or should) your race day requirements/performance/conditions. Attached is my power profile from todays Criterium in Allentown, PA, a 25 lap (.85 mi/lap), with accelerations lasting approximately 1 minute followed by a bit of a rest of 1 minute. You can see when I was ON, the power requirements averaged approximately 250 watts and when I wasn't I was usually pedalling/pushing about 175 watts which is basically a warmup power. My Functional Threshold Power (FTP) is 270 watts so I was pretty close to Threshold Power...which coincides with my Threshold Heart Rate of 174 bpm.
If you're racing (particularly Criteriums) you should be fitting these types of intervals into your training plan at least once a week. That is, in addition to racing. You can either do 1 min. ON at L4 and 1 min. OFF at L2 for 50-60 minutes, or do 1 min. ON at L5 and 1 min. OFF at L2 for 25 minutes..if you're short on time or just want a good VO2max workout.
The idea is to replicate RACE DAY during your training.
BTW, I sat-in for this entire Criterium...finishing mid-pack. I tried to improve my position on the last lap but couldn't because the pace was too quick. (see graph)