In theory the stroll between Gares Nord and Est is about 5 minutes - IF you know how to go - the most direct route is via the staircase that comes down on the west side of Gare de l'Est but this is not obvious unless you know the route - many would head down to the big boulevard and hook around - doing three sides of a square and this could take much longer - especially with luggage or pets or tykes in tow.

The two stations practically touch each other - wonder why there is not a dedicated walkway between the two that goes under the myriad of tracks?