"How much calories does a man need on average per day to carry 17 kg for 19 miles/day?"
There are lots of post on this, but when sedentary then very approximately men about 2000, women 1800. That is when doing nothing at all. When backpacking hard, such as going up and down mountains with a pack that includes 30 lbs of food, expect this to be 4000-6000 cal a day. Lots of people such as myself, who don't mind loosing a few lbs are willing to do longer trips with 3000 cal/day, but if you are planning on doing 20 miles a day with a heavy pack this may eventually cause you to "bonk", or at the least fill your dream with images of food. Especially if you are already thin.
In the end you have to have the experience to know what works and is comfortable for you to answer that question. But 2000 is too little.
Also, what about water? Much more of a concern. specially in southern Greece, and especially in the mid to late summer you may have a hard time finding surface water. You will have to plans that much more carefully, and you may have to carry a lot of water in places. Just another reason to reconsider your plan/need to carry so much – you may need to carry a lot of water too.
By the way, while Greece does not have a cultural tradition of tramping that I know of, what it DOES have is monasteries pretty much everywhere, AND a tradition of letting people stay overnight for a nominal fee. Yet another way to do things in a way that seems more adapted to the local parameters.

