This was an appropriately light race-week interval session: 6.2 km in about 40 minutes, with a low 142 bpm average heart rate and no reported pain or discomfort.
The heavy-leg feeling is much more consistent with sleeping less than four hours than with a fitness or recovery problem. The session itself stayed controlled and the heart rate settled back down during the final easy kilometres.
With five days remaining before the marathon, the priority is now sleep and freshness. Thursday should keep the legs moving without creating any meaningful training load.
Takeaway. There is no fitness problem to solve. Prioritize sleep, keep Thursday easy, and use only a few very short relaxed accelerations if the legs feel naturally springy.
Next step. W16D4 Easy Run. Short race-week easy run with optional relaxed accelerations to keep the legs awake without adding fatigue.