Hi Brian,
My first thought was: "59 minutes between
Gare du Nord and
Gare de Lyon is tons of time". I just did that trip yesterday, albeit at rush hour, with a bit of shopping in between. I arrived via
Eurostar at 17:25, took Metro Line 4 till Les Halles, exited the station, realized I should have stopped at Chatelet, walked 8 minutes to my favourite chocolate shop for a box, walked back to Les Halles station, took Metro Line 14 to
Gare de Lyon and I arrived at 18:10. In other words, if you did all the tomfoolery I did yesterday, you'd still have time leftover to make your connection.
I'm just about to update the
Gare du Nord photo tour with a few more photos that might help you remember the layout. Once you exit the
Eurostar train there will be a set of stairs diagonally to the left. Down that flight of stairs are two new green
Billeterie Ile-de-France ticket machines where you could purchase your
Metro/RER Ticket t+ (given that you have 1,60€ in change or a smart chip credit card). These machines are usually quiet as they're not in the most obvious spot.
The RER D train goes directly from
Gare du Nord to
Gare de Lyon. Check out my article on
using the Paris Metro route planner which could give you some idea of timing.
Best regards,
Ben