6 week to get fit....?

Hi All

In just 6 weeks time I will be flying out to Kenya to undertake a 400km cycle across Kenya for charity.
I have been training for 18 months for this challenge and am alot fitter than I was when I started... however I am now in panic that its just not enough to undertake the challenge ahead.

So.... is there anyone out there who can help? Can I increase my fitness further in just 6 weeks? What do I need to do (am a member of a Gym and a Spinning Instructor - I spin at least 3 times a week), and what do I need to be eating (I haven't completed my diary over Christmas as have been away from the computer, but what foods should I be consuming in this last 6 weeks?

Any help appreciated - please help me build my confidence for this challenge and inprove my fitness.

Thank you!