Getting from the Gare Saint Lazare to the Gare du Nord
 

I will be coming back from Normandy and then catching the Eurostar to London. Is the metro easy to navigate from one train station to the next.

RER E is nonstop, walk a couple minutes from St Lazare to pick up RER E and get off in one stop at Gare Magenta, walk a couple blocks to Gare du Nord. Maybe 15 min in all

Metro requires a connection (line 3 or 12 to line 4) but leaves from Gare St L and arrives at GdN. A couple minutes more than RER but less walking overall. If you are not toting a lot of luggage should be pretty easy.

And there's always taxi/uber, but you are at the mercy of traffic.

https://www.seat61.com/Paris-metro.htm [scroll down to your specific route]

The above link covers all of the issues. With luggage, I prefer to taxi. The metro is quick and cheap, but there is walking at both ends, you have to buy metro tickets, etc.

The RER E Magenta station is not "a couple blocks" from Gare du Nord. It is IN Gare du Nord.

I think he's thinking of Gare de l'Est. The exit from RER E is a bit of a walk over to Eurostar, but not that long. Magenta isn't a rail station (gare), it's just an RER stop/

Yes, RER stop is in GdN, just that it is a bit of a hike upstairs and to the other end where Eurostar is located. Should have have phrased it "the equivalent of a couple blocks"

Magenta has the most massive number of escalators to be seen in Paris, so the term "'upstairs" should not engender visions of struggle.