During my early thirties is when I hit my power drinking stage. During my mid thirties to late 30s I started trailing off since you physically can't drink 36 beers anymore. That's when you switch to liquor since guess what, you can pack 36 beers into a fifth and only 12 beers. During my early 40s is when my body started waving the white flag. I lived with physical pain, ulcers, and just feeling generally shitty.
Once you're alcohol free for a few weeks you feel so much better. I had to actually stop, and sit still for a while and think to myself, "You know what? my guts don't hurt anymore. Is that for real?" Sure enough, you almost forget that your body was in pain almost constantly.
My last drink was a bunch of margaritas with extra floaters. I felt horrible, and that was the launchpad coupled with a few grams of some schwag that got me sober for a year and well two months now.
The sleep is a big one, I was a big after work drinker, and when you have to pack all that drinking into the space of a few hours then pass out and head to work the next day, you're lucky if you get a few fitful hours of sleep.
That's one of the BIG improvements you'll get almost immediately, there are of course other benefits, but that'll be the big one you'll notice right off the bat of a few weeks!