IO540 - Think you're being a little harsh here. There's a world of difference between navigating round well-trodden routes and airspace as you're taught in the PPL, and leaping into a long cross-country as he's planning.
Rishy - you DO have all the skills you need. You just need to realise it - that was my major stumbling block. Break the trip down into small legs of about 50 miles, and take one at a time. Perhaps plan the first leg to coincide with a nav point / route you covered during your training.
You'll soon realise that if you know how to navigate between A-B, you know how to go on from B-C and C-D. There's no fundamental difference. It's all confidence. And going with an instructor is a good idea, but don't get anything rubber stamped - just go with your plan. You're P1, so get used to it !
And yes - take some oil just in case.