I have seen runaway processes that not only run a long time, but also start to consume CPU resources.
As far as I am concerned, any runaway process will eventually take over as much as it can.
I had an issue just today with a process that was use 200% of 2 CPUs on my server. It had been running for 91000+ minutes. It wasn't a zombie or anything like that. It just "ranaway". I killed it and restarted, then everything was okay.
I can't really explain why these things happen. Maybe they get dropped by the OS or lose some ability to be managed by the OS and then instead of being "timesliced" and allocated CPU resources, they start to take over resources without permission.