Rowbare got it. Just a sander. Orbital type sanders are better than other types for leaving a fairly flat surface. If you are real good with a belt sander, you could use it too, but not really recommended. It can just as easily screw it up worse.
You can also use scrapers. A good card scraper would work. I have a jointer blade mounted in a good paint scraper and that works fairly well on aluminum too. Nothing will get you a surface like milling or grinding would though. Only close, which may be good enough for now, as I mentioned.
The THK 25's that I have are brand new. The rails are as well. They will not fall by their own weight. They are preloaded pretty well. With a little added weight, they do fall by just gravity. If extra weight will make yours fall, it may be good enough, but because there may be a misalignment, you might wear the bearings out quicker. The closer to perfectly parallel and coplaner that you can get, the longer all the parts will last.
What you have now is an egg. Just make breakfast. With a good breakfast in you, you will be able to build a better chicken later.
