The length of the corridor will depend on how big a cruise ship you are on Thomson Celebration not so bad but Queen Mary 2 a lot longer but they are wide enough to take two people walking side by side with still a little space left. There are plenty of stairs on all the ships so no problem there and they normally are wider than you might find in most hotels so no need to be to concerned and enjoy your cruise.
I would take a short 'taster' cruise of 3 or 4 days, go for one of the newer breed of ships, say post 1996, avoiding the older generation of cruise ships. The modern vessels are more floating resorts than ships, & lifts are often the 'scenic' plexiglass see through type looking into the main atrium, should you wish to avoid the stairs......Good Luck Wilba.
Why not book a cabin that is opposite the stair/lift lobby. this is usually a wide open area and then you will avoid corridors. Look up the deck plans of a ship you want to holiday on and you will see that there are plenty of cabins in that position. choose and book. have a lovely time.
Plenty of stairs but corridors could be a problem. Try booking a cabin right at the end, adjacent to a stair well. If you opt for a balcony then I think your problem might be resolved. Please DO have the courage to go for it as there is no finer experience than your first cruise.
Fine advice to go for a more modern, larger ship, BUT check your destinations and whether you will have to be tendered in. I find this the most claustrophobic part of a cruise. It would be a shame to miss great ports of call because you can't just walk ashore. Small ships are more likely to be berthed, have shorter passage ways and fewer decks. Any ship that has been a ferry in a previous life will have quite low ceilings, probably something else to consider.