Hi Joel,
the GTDish way would be to move the "buy milk" into a folder "errands"
and/or context "@groceries". Then whenever you go to the supermarket,
you look up that context as your shopping list. If you tend to forget to
go to the supermarket, you can add a recurrent task with a reminder to
MLO or to your calender.
Generally, the idea of GTD is to not schedule too much. You put
everything that absolutely needs to be done at a certain time (like
meetings) in your calendar. The rest remains flexible.
You pick the things that you want to do based on importance, urgency,
and current context. You also use other criteria such as available time,
you current mood, energy. (MLO has properties for "min time" and "max
time", and you can create contexts like "@tired" "@focused" for things
that can be done with low energy or that need high focus).
MLO helps you with views like "Next Actions by Context" to pick the
right action. If you like, you can pick the things you want to do today
with a star, and then work your day in the Starred View.
-- Chris