If you miss a day and don't open Mend at all, you'll pick up right where you left off. So no need to worry about missing a day. If you did open Mend and a training loaded, but you just weren't able to listen to the training that day, you can flip back to that day and complete it.
Simply navigate to the journey screen by clicking on the map icon in the top left corner navigation bar.
Simply navigate to the journey screen by clicking on the map icon in the top left corner navigation bar.