I don’t think Xert takes into account future one-off rides in giving its guidance, but it it’s an interesting idea.
If that pattern of a long Sunday ride is repeated, Xert will start to take that into account though, also for other days’ recommendations. So if your history has recently included long Sunday rides, that will be reflected in the Saturday advice already
If not, it sounds like you know you want to ride endurance (to save your legs), so then it’s just a question of how much you can handle and still feel ok for your ride tomorrow, based on experience…? 1 hour, 2 hours…
Then you can use the workout filter for endurance focus, low difficulty, and your time constraints.
You could then play around with the planner to see what your form and status will be after the long ride (add the workout and a manual activity for the long ride) eg to make sure the combination doesn’t send you into the red for too long.
A 125km ride on Sunday, and you want advice for a ride on Saturday? I honestly think your personal experience with these types of rides would be more valuable than any advice Xert could give you. But if you wanted Xert’s advice…I think if you click on Sunday in the planner, it will show you your training deficit as of that date. Then if you’ve done a 125k ride in the past and know it’s XSS, just subtract that XSS from that training deficit. You’re left with the XSS for your Saturday ride.