To get to Gare de Lyon, it appears that you would take The RER B Train to Châtelet Les Halies Station then change over to the #1 Metro Train out of Châtelet Station. However to do that you have to walk between those 2 stations. The walk as I see on the maps seems to be quite a long walk. Is that the case? Our ultimate destination after arriving at CDG is the Marriott Courtyard Gare de Lyon which is a short 4 minute walk from the Gare de Lyon Metro Station. It’s the walk between the above mentioned train stations that appears to be quite a distance. Am I seeing this correctly? How long is that walk between those two stations please?
Also it appears that a single Ticket Mobilis for Zones 1-2 for only 8.45€ is all that is needed to accomplish that. Would you agree? Will be in Paris only 2.5 days this leg of the trip.
CDG to Gare de Lyon by train can be done either by RER B + RER D, RER B + RER A, or RER B + Metro 1.
Any of these choices will result in similar walking times (around 6-8 minutes total). The only difference would be where/when one does the walking. The transfer walk from RER B to Metro 1 (requires one to walk between Chatelet les Halles and Chatelet stations, about 5 minutes) will be longer than the transfer walk from RER B to RER A at Chatelet les Halles (about 2 minutes). But the walk from Metro 1 Gare de Lyon to Marriot Courtyard is shorter (about 2 minutes) than RER A Gare de Lyon to Marriort Courtyard (about 6 minutes). So, in the end, you'll walk a similar amount of time regardless of your choice. If you don't look forward to wrangling your luggage between turnstiles/fare-gates inside of Chatelet les Halles/Chatelet, I'd recommend doing RER B + RER A, which has zero turnstiles/fare-gates between that transfer (you're already inside the fare-paid area when making that transfer). Otherwise, you will be passing through fare gates going from RER B to Metro 1, which is at an adjoining Metro station (Chatelet, which is different than Chatelet les Halles, the RER station, although the two are attached via tunnel).
Regarding tickets, CDG to Gare de Lyon requires a 1-5 zone Ticket Mobilis. But, unless you're doing a round trip CDG-Paris the same day or CDG to Paris plus 4 Metro/Bus/RER rides that same day, it wouldn't be the optimal choice to buy an unlimited use day ticket like Ticket Mobilis. If you do plan on making another 4 rides the same day you arrive at CDG, then Ticket Mobilis 1-5 zone would actually make sense and I'd recommend it. Otherwise, buying a single-use RER ticket from CDG to Paris is usually the most cost-effective option for CDG to Paris Gare de Lyon. Then buy/use single-use paper Metro tickets for any remaining trips you need that day. If you plan on making 4+ trips the next day, Ticket Mobilis 1-2 zone is worth it / optimal.