Hi Micael.
For a first time route, you might find trying to make a 150 mile long route a bit too overwhelming and may soon will realize that it will take a looooong time and a lot of work to fill in all the various scenery and such to make it all look visually appealing, so you might consider making a smaller route for your first go at it.
For the first fully-working route I made (King's Arm Highway:
http://www.rryard.com/kings_arm.html ), the overall route length I believe is roughly about 18 to 20 miles in length (taking into account some of the side paths), and that took me around 400 hours of time to complete. That's also including several scenarios I made to go along with it, which can take quite a bit of time in themselves to make.
For building a larger longer route (which I'm currently doing myself now:
http://www.rryard.com/western_wildwood.html ), maybe try it in smaller chunks first, 15 or 20 miles in length to start with, fill in some details, and see how it shapes up from there. If after the first 15 or 20 miles completed, and if you want to keep expanding on it, then try to add on another 15 or 20 miles, and so on.
Once you make some good progress on and are feeling good about how things are shaping up, you should consider having a couple of people to help beta-test things and give you feedback in how to improve on things or point out some mistakes or errors you may have missed. Sometimes it can be hard to take criticism of one's work, but taken in a positive and constructive way it can be invaluable.
It's your route, and every route-builder has their own approach of how they like doing things, and if you are willing to put in the time and effort to make such a huge route (150 miles) then go for it. Being a fictional route, you can more let your imagination run wild and not be limited or restrained as much, so be creative and have fun with it. In route-building, sometimes one can "run out of steam" so to speak

at times, so remember to take a break once in a while, then come back and attack it again refreshed.
For good ideas and inspiration, sometimes sit back, relax and drive some trains on some of the other routes. This can help clear the mind a bit too, is a little break from the route-building, and often times you'll see some things that you might consider trying similar on your own route.
Post up some screen-shots sometimes and let us know how progress is coming along.
Feel welcome to ask for advice in the forums, and I'm sure people will be glad to give input as they can.
Good luck!
