This was a strong threshold run after the official 21K: 13.4 km in 1h18, with normal legs and a solid effort despite drizzle, stormy conditions and some wind.
The session was not destructive, but it did carry real quality. Several early kilometres were close to threshold or faster, and the second half settled back down well.
There is no need to compensate for the Garmin long run that was replaced by the official half marathon. The 21K was the workout that day. The correct adjustment now is to protect the weekend and execute the next long run with discipline.
Takeaway. Do not pay back missed kilometres. The official 21K replaced the long run. Keep Saturday very easy, then use Sunday as a controlled marathon-specific long run without extending it.
Next step. W13D6 Easy Run. Short easy run before the final big marathon-specific long run. Keep it relaxed and deliberately light.