Hi Luke....welcome to the Forum
I would imagine that a car of this age with this mileage has probably not gone out of town much.
I have had similar cars in the past that also have suffered from low MPG and performance and with longer runs out of town and gradually increasing the engine load and revs have managed to clear the engine out with much better results on performance and economy.
It helps to add a tin of Redex or similar to assist the clean-up but otherwise, it is a case of bedding the engine in after poodling around town for so long
Let us know how it all goes