Translation:I hate running.
Actually, it makes a verb an actual noun or some other action that usually ends in -ing that doesn't represent the gerund form of the verb- in this case it makes the verb "to run" mean "the act of running". I assume you can use it for other verbs to mean similar things. "I hate swimming", "I hate driving" etc.
IT is the shorter version of saying KOTO (事）like, 走るのは is the same as 走ることは（走る事は）To make the verb into a "something" or in other words to make it a noun.
For people who are asking, and for a little insight. As a lot of people said, の is used to change the verb into a noun. Instead, you can use こと when you want to be more polite. Good thing is that they work in a lot of way, as long as you want to say sentence with a verb as a noun in English or in infinitive form. たとえば : I can drink this / これを飲むことができます。
Other thing, you can transform an adjective into a noun. Once, I was in Ghibli museum, and I heard a little kid said : くらいのこわいね. In proper version : くらいことがこわいですね. It means : The state of dark (くらい dark as opposed to light, talking about a place) is scary isn't it? (ね is asking for the approbation of the listener, here it was his mother).
Hope it will help some people!
の turns the verb "run" into the noun "running", as in "the activity of running". が is the normal subject marker. This allows us to use the verb hashiru/to run as a subject or object that we can talk about in a sentence, like in this sentence "I dislike running".
Adding a の turns a verb into a noun. Hashiru (走る) means "to run." Hashiruno (走るの) means "running."
There is a very slight difference between the two, but the easy answer is that either is fine.