Use the AfterUpdate event of both your From and To boxes to lookup the
distance in the table and assign it to the field on your form.

This kind of thing:

Private Sub LocationFrom_AfterUpdate()
   Dim strWhere As String

   If Not (IsNull(Me.[LocationFrom]) or IsNull(Me.[LocationTo])) Then
       strWhere = "([LocationFrom] = """ & Me.[LocationFrom]) & _
           """) AND (LocationTo = """ & Me.[LocationTo]) & """)"
       Me.[Distance] = DLookup("Distance", "Distances", strWhere)
   End If
End Sub

Private Sub LocationTo_AfterUpdate()
   Call LocationFrom_AfterUpdate
End Sub

Notes:
=====
1. Adjust the code to match your actual field names. Note that "FROM" is a
reserved word in SQL, so not a good name for a field. I've therefore
suggested LocationFrom and LocationTo as field names.

2. If the LocationFrom and LocationTo field are Number fields (not Text
fields), lose the extra quotes, i.e.:
       strWhere = "([LocationFrom] = " & Me.[LocationFrom]) & _
           ") AND (LocationTo = " & Me.[LocationTo]) & ")"

3. Calling the one procedure from the other is easier to write and maintain
that duplicating the code.