of Theoden's battles?

So ... I was recalling a line from the movie (I know its precarious to begin any assumption coming from the movie) in which Théoden is talking to Aragorn and Gimli I believe, and he says something to the effect of "I have won many battles ... a king knows how to protect his own keep." I believe there is something in the book to the same effect and I was wondering who you folks think Theoden had been fighting in his decades of rule. There are other comments from the men of Rohan like "war is ever on our borders" but I'm wondering where exactly, considering we hear of no major battles in (or even near) Rohan until all hell breaks loose. Here is my point:

We are lead to believe that much of S. Eriador that lies NW of Rohan, East of the Misty Mountains is at best sparsely populated and even where populated it is not well ordered (especially not ordered enough to want to fight a kingdom of tall, strong, horse-men). We know that until his betrayal Saruman, to the North did not engage Theoden. To the South was Gondor, obviously not a threat. North was Fangorn and further up lay Lorien both of which held no enemies. East, S. East was the River, Emyn Muil and directly east was the Dagorlad/Dead Marshes. It seems that we can rule out most of the areas surrounding Rohan as without any real threat to Theoden.

I am obviously overstating the case a little bit ... there were orcs comming south out of the Misty Mountains and orcs/men potentially coming from Mordor, SE. But we are given reasons to believe that Sauron had not been waging war against Rohan. Therefore my question remains who were the Rohimim warring with that Theoden (either king or crown prince) was out fighting?

The whole business sounds a bit fishy to me ... maybe there we just a bunch of very small skirmishes with surrounding Goth/Vandal types, maybe Theoden was just trying to make himself sound a little more bad-*** than he really was, maybe I'm wrong and there is nothing in the book like that ... but Id like to hear some other opinions so we can at least chalk another one up to PJ and the writing crew.