To get to Gare du Nord, with luggage it is a ten mins. walk. When you get to end of the train platform, don't go straight out into the main central entrance of Gare de l'Est, instead turn right and you should walk right by the waiting room, once outside on the sidewalk, walk up the stairs..it's little taxing if you're dragging luggage...but you won't be the only one at that time in the morning. When you get to the top of the stairs, go straight...there are signs pointing to Gare du Nord and you can drop into the two hotels there to ask. Those hotels are both two star and three stars; after that Gare du Nord is about 3 blocks on your right hand side.
Once in Gare du Nord the Euro-Star check-in is upstairs, not on the ground floor...signs are everywhere, can't miss it.