Depending on the priority and intensity of the race, you will do one of the following:
- For a low priority and/or lower intensity race, you can just replace the long workout with the duathlon, and then proceed as normal
- If the event is a higher priority and/or high stress, you can:
- Replace the workout prior with a race opener
- Replace the long workout with the Duathlon
- Schedule a rest day or active recovery day following the event
I hope this helps!