TrainAsONE creates your plan with the best training volume and mix of workouts to promote the most efficient improvement in performance whilst balancing a low risk of injury, and conforming to your training constraints.
Consequently, having a simple ‘long run on Saturday / Sunday’ configuration option does not make complete sense. If your longest run of the week is calculated to be 75 minutes on a Tuesday (and your constraints state that you can train for 75 minutes on Tuesdays) then there is no logical reason why this should not be allowed (and that the 75 minute run ‘moved’ to Saturday / Sunday, which in turn would make a less beneficial plan).
The underlying desire of having the longest run on weekend becomes subjective, personal, and no doubt changes at different times.
Thus if you find yourself in the situation that you would like to coerce the system to schedule your longer run at the weekend you can (artificially) limit your time availability for the days you would prefer short runs only.
This is best done from your Training Settings page, accessible from the Settings menu (the ‘cog’ in the top-right corner of your screen, where you would set maximum run durations for your week day runs. This may still leave a speed-session scheduled on weekends, in which case you could also mark these days as easy runs only (select custom from the schedule workouts option, and then tick include easy workouts). We recommend not keeping this setting permanent, as it would cause fast variants of the long run never to be scheduled.