If you're afraid about surviving Economy, invest in Economy Comfort in the front of the plane. It's 150 (?) USD extra and you'll certainly not have any problems with the legroom. Or invest a few hundred USD more and upgrade to business. Not knowing loads on your flight (naturally), I find it hard to imagine you would be able to "spread" across the back of the aircraft. I haven't flown such flight TATL in the last 5 years...