I use this type of workout in marathon and half marathon builds extensively. These are also effective at other distances. What changes is the specificity of the pace varying to the distance we are training for.
Why and how:
Alternations improve our ability to handle lactate.
We vary the pace so that it is slightly faster than lactate threshold and slightly slower.
The benefits of this type of training comes from the work done at the slightly slower than lactate threshold. The work we do above lactate threshold causes our blood lactate to rise. When we transition to the slower pace, we are teaching the body to clear the lactate.
Examples:
Marathon training
Early block: 8 to 12 km continuous as 1000m at slightly faster than LT - near 10km effort, RPE 7.5 to 8 / 1000m at MP, RPE 5 to 6
Later block: 2 to 4 sets of 3000m at MP, RPE 5 to 6 or slightly faster / 1000m at 10 seconds slower than MP.
Advice:
Pushing the pace defeats the purpose of the workout and can lead to the slower sections becoming too slow.
Rhythm runners or those with a strong aerobic engines tend to do well in executing these sessions.
Runners used to pushing out fast intervals with decent recovery tend to struggle with these sessions.
Integrate alternations into your training plan every three to four weeks.
Stick to about 10% of your weekly distance for alternation volume in a session.
