Coach's Call Z2 Endurance 20min The 20-minute rung of the Z2 family, and the exact argument z2-endurance-25 makes one band up: aerobic base needs an option wherever quality has one. Below 25 minutes the authored endurance floor ran out, so every path ending in an EASY day fell to recovery-20 while every path merely CAPPED at sweet spot reached sweet-spot-2x6-short and got real work — which inverted the day. 79 monotonicity violations in 20,000 fuzz states, where dropping TSB 8 points, reporting a poor night, or landing on the back-to-back day-2 ceiling moved the athlete UP from a recovery spin to sweet spot. Three doors, one cause, one entry. Unlike sweet-spot-2x6-short this needs no relaxed dose floor: a pure Z2 session sits entirely below 75% FTP, so 20 minutes meets the category's full 1200s in-band floor exactly, with a real 3-minute warmup and a 2-minute spin-down still inside it. bike