You did?
I'm still at lvl 21 with 40 hours of gametime.
Seems to take a really really long time for me.
Does difficulty level affect exp gained?
I've just never reached a point where I wasn't constantly gaining XP. When I was mainly just exploring, every time I came upon an enemy, it was
always a group of enemies. If I ran into a few Radscorpions, you could be sure there would be just as many Giant Radscorpions. If I ran into a Lakelurk, it was guaranteed there would be handfuls more just around the corner. And so on and so on. Hell, I've never ran into a Deathclaw without also running into 6+ more at the same time. The game just kept on sending me opportunities to earn hundreds of points in XP from a single encounter with enemies.
Then when I started doing quests, that wasn't much better. You still get hundreds of XP for completing a quest, which in my opinion is pretty ridiculous. It was fine in Fallout 3 where there were fewer quests, but there are tons of quests in NV which translates into tons of opportunities to earn hundreds of points of XP.
I constantly leveled up and didn't notice any real slowdown until I reached the 20s. I'm level 25 currently with 30 hours in but even now I seem to gain XP disturbingly quickly.
Oh and no, you earn the same amount of XP no matter what difficulty level you're on, which really only makes things worse.