By now you are most likely finished with your tempo (AKA cruise interval) work and are into the phase of training that requires you to develop your muscular endurance. Muscular endurance is gained by working to increase your power at your lactate threshold (AKA functional threshold and anaerobic threshold) as well as the duration of that effort.
These intervals are meant to build that lactate threshold (LT) power. Once completed, you will have a 45 minute to 1 hour killer effort in your arsenal. You’ll also use this very important number to gauge all of your zones because, from the standpoint of information, LT is ground zero for an endurance athlete.
Do these on a stationary trainer or outside on a flat/false flat course. These are also great to do while going uphill if you are lucky enough to live close to a mountain with a long enough climb:
LT10: 3×10 minute intervals at lactate threshold with 3 minutes off between them
LT15: 3×15 minute intervals at lactate threshold with 3 minutes off between them
LT20: 2×20 minute intervals at lactate threshold with 8 minutes off between them
LT25: 2×25 minute intervals at lactate threshold with 8 minutes off between them
LT30: 2×30 minute intervals at lactate threshold with 8 minutes off between them
LT45: One 45 minute interval at lactate threshold
Once you know your heart rate and wattage at LT you can keep an eye on your monitor and adjust up or down to suit. As an example; if your LT is 165 BPM’s/300 watts and you see that on a particular day your heart rate is less than 165BPM’s at 300 watts then increase the wattage. When that happens you’ve just increased your LT power! Note these changes and feed them into your program going forward.
As with your base and tempo workouts, make sure to complete the duration properly before moving to the next. Slowly and methodically building to your objectives will help prevent burn-out and injury. It’ll also make you faster!