i had to check your profile to see where you are from.I assume you're from UK, right?
So, here are some useful info, traveling from UK to Greece.(i think you'll find more pages like these on the net, this is just for a start.)
by train
http://www.seat61.com/Greece.htm
by ship
http://www.voovit.com/country/shipping_greece.htm
I do agree, with the first user above, that this will take you some time, but if you look at it on the bright side, you are a traveler from the moment you walk through your doorway! And if you choose a trip by sea, then you'll have a good time on the ship, they usually have shops, cafeterias, bars, etc. So, you won't notice how fast the time will flow.
Have a great time!
(and don't worry, your question has been asked many times in the past, in our section, you could use the search bar to find some resolved questions about alternative way of traveling)