I am a dyed in the wool city-girl. I start to get claustrophobic in the countryside. Why the hell would anyone want to live in the middle of nowhere is beyond me, and it really does freak me out. Give me the smog, the noisy traffic, and the crowds. I haven't lived in London, but I have lived in NYC, and that is one hell of a big city.
As BH said, it is all about stamina, and drive. But I do find I am more motivated in the city, than in the country. Different rhythm and flow to things.
You have one life, and if you are lucky enough to have a ready for you job upon arrival in London, then you are half way there. If you need a break, this is what day spas are for. The Sanctuary in Covent Garden is lovely. Go for it, it will be challenging, but this is how we grown as individuals. Good luck.