Ha! Only tens of thousands of LOC? I've worked in places with millions of lines of spaghetti code. The reality is that most business software that's been around for a while looks like this.
If the environment itself is good, and you like the people you work with, then you should see it for the great opportunity that it is; learn how to slowly evolve a large software project for the better. There's none of the stress of trying to figure out the "right" architecture, no worrying about making bad decisions. You can just chip away tidying things up.
Things that are probably a bad idea: Trying to rewrite (will almost certainly fail), Blaming others (it's no one's fault, it just is what it is), Walking away (negative signal for future employers).
If the environment itself is good, and you like the people you work with, then you should see it for the great opportunity that it is; learn how to slowly evolve a large software project for the better. There's none of the stress of trying to figure out the "right" architecture, no worrying about making bad decisions. You can just chip away tidying things up.
Things that are probably a bad idea: Trying to rewrite (will almost certainly fail), Blaming others (it's no one's fault, it just is what it is), Walking away (negative signal for future employers).