Slash, like many guitar players, when it comes to keep notes going like your talking about, will first determine where it is that they are going.

Let me explain this a little better ;)

It usually comes about when you say "how can I connect note X with note Z, when Z is clear over their?". Then you start learning some proper technique to get the fingers working. Let's say that you want to play 80 notes in perfect connection. That's a lot harder to keep going with than 20 notes obviously, so you break it down into parts. You master the first 20 notes, then the next 20, and so on. This works for memorizing the notes and getting the muscle memory down, but then you actually have to connect the notes at there "intersecting" points. The best way to do this is to start really slow, and that will ensure that you can go from one note to another smoothly. If you keep playing it at the same rate of speed for a while, you'll find that your fingers are like "all right, I gotta already!" and they will start to progress on their own. After a while though, you will have to push them a little bit. Don't over do it, but you have to then show your hands their true potential.

In terms of improvising, the more that you have worked with these concepts, the more that it becomes a part of your approach to playing, and it then can be done off the top of your head, but this only comes from jamming a lot. Practice is necessary, but you have to jam off the top of your head too.

Slash uses a lot of Pentatonic scales, which are a great place to start, when it comes to connecting the dots. You can take the Pentatonic Minor scale and move it around the neck, which will take it between Major Pentatonic and Minor Pentatonic.

Hope this helps, if you've got anymore questions - let me know.

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