"He still comes around"
I don't think you've made a clean break from him and that might be an important thing for you to do, to feel. I suspect you aren't sure about leaving the relationship. That's understandable when it's been a long one and has been on/off. I agree that it's perfectly fine to realize something isn't for you and decide to do something else.
I think the overwhelming distance of your trip is, in your mind, a metaphor for the distance you have decided to place on this relationship and it is understandably unsettling, as new experiences can be.
It will be okay. You are doing what you want to do. Instead of thinking of the distance of your trip as one big stretch, you could break it up and think of smaller parts of the trip.
You have some very wonderful friends who are coming to help you with this! Can you picture it as friends having fun tooling along, seeing the sights that you missed travelling by plane? Have you planned some stops that would be interesting and fun? Got the camera ready? Make the most of the trip as a celebration of realizing what you want and the courage to do what you need to have what you want!!
Have a fun trip and let us know how it goes!
ECHOES