Have you ever asked yourself this question before booking your flights? I encounter many tourists that are surprised by how large this city is and complain about the time spent going from, let's say, Oxford Street to Greenwich.
My best piece of advice here is to take a map (Google maps works just fine), choose what you want to visit and see how long it'd take you to arrive from one attraction to the next one.
You may think that everything is too far away, but when you come to London, you have to change your perception.
Basically, anything that is less than 30 minutes away by public transport is considered to be close. 30 minutes to one hour, average; and over one hour, far away.
Please bear this in mind or you will feel like you're wasting your holidays in a smelly underground carriage (actually, you are, same as Londoners waste months commuting). But still, it is worth it.
