As someone who's already watched the whole series, I can confirm you've made a great decision. It's funny what you say about being reluctant to watch long series; I felt the same way at first, but once you're immersed in that gritty, industrial Birmingham, you won't want to leave. You're absolutely right about the atmosphere: it's incredible how they manage to make you almost smell the coal, the smoke, and the misery of the era through the screen. Visually, it's a marvel from beginning to end.
Also, get ready because the soundtrack is on another level and gives it a really cool, modern touch. Enjoy the ride—you've got a quality binge-watching marathon ahead of you!