You're right, it is a lot to write in one month, and it is hard to do. I've done NaNo several years now, and I've never the reached 50k goal.
However, I did get a lot written each time, and that's what really counts. It doesn't really matter if you hit 50k or not-- the point is to encourage and inspire writing.
Of course your 55k word story took over a year to write. A lot of that time is spent editing, rereading, and revising the story. NaNo is all about word count. Just get words on to the page. It's a time when you don't have to fret and fuss over getting the exact right word for that one scene, which is hugely liberating to many writers-- especially perfectionists like myself : )
I would recommend participating even if you know you can't get to 50k. It's one of those 'it's about the journey not the destination' situations.