I learned to speedrun the game by just playing casually through many, many NGs. Over time I know attack timings, parry frame data and roll frame data which at times is used in a run. There are managing stats, what you buy, what items like estus shards, titanite shard and chunks, etc. I have two characters that have over 999:59:59 time, mostly from PvP and just going through multiple NGs just to get a crap ton of titanite slabs for weapons I want to try in PvP.
Dark Souls, in essence, isn't difficult. The learning curve and finding the trolls and routes is the tough part, and the lag from playing online is just something you have to get used to. Similar to Melee, where over time you learn what to do, what not to do and how to punish based on the character they use. In Dark Souls, the character is pretty much the weapon the other person is using. To brag, I can guess what weapon you're holding that is invisible just by your character's hand position.
My PB as I timed with my phone ever was 2:30:43** RTA, but since I have no gameplay footage of the run there is no true proof of it existing.
EDIT: Time was wrong, that would beat WW and is pretty impossible