6 × 3.5min on / 1.5min off

This is a classic VO₂ / threshold-VO₂ session: a 15-minute warm-up into a 10-minute steady block, then the main set, six 3.5-minute efforts off 90-second floats. The on-reps sit at and just above threshold power, around 455–480 W against a ~465 W running threshold, near 3:35/km, so this is top-end aerobic power work, the gear a 50 km build rarely gets from long aerobic days. The 10-minute steady primer is the tell: it pre-fatigues the system so the reps start from an already-elevated place, closer to race-day cumulative load.

Execution was controlled and even. The reps clustered around 3:35/km before a 3:28 close, with heart rate climbing in a tidy staircase from 171 to 176 and never spiking. Time in Z4–Z5 totals roughly 38% of the session, exactly where this workout should live. Power tracked pace cleanly with no fade in the back half, and the floats held a true 90 seconds. Aerobic decoupling was minimal, the second half kept the first half's efficiency, which is the signature of a session finished under control rather than emptied out.

Two things to sharpen. The first reps were a touch conservative before the legs woke up on the last one; distributing that effort earlier would buy more honest time at VO₂. And for a 50 km build specifically, the next progression is duration over intensity, the same shape at 4–5 minutes on would push it toward the sustained strength the distance actually demands. As a top-end stimulus inside an aerobic block, though, this is well-judged work.