It's a very strange world for me because I have a lot of theory already and a full mastery of the fretboard, but I don't have ready access to the basics of jazz language in the way that I do for, say, rock guitar. (Obviously there is very little improvising in classical guitar.) This is partially because, well, I really can't stand the sound of the saxophone. That's pretty limiting when trying to pick up ideas from the great jazz players. Also, I tend to love the piano most of all in jazz and it's not easy to get playable ideas from those guys because they have so much to work with that can't be played on the guitar.
At any rate, this will be a long process, but I am already enjoying the challenge. And the challenge is both getting used to a world I don't know and also digging my way through the horrific pedagogy that comes in some of these jazz texts. Here's a quote from Jazz Guitar by Larry Coryell:
The first solo phrase (in bars 2-4) draws from F Mixolydian; just after that, the four notes in the extra first half of bar 4, E-G-D-C, come from C Mixolydian. When I got to bar 9, I played a C# leading tone into a Dorian ascending scale, which is connected to the C Ionian mode or C major scale...I used the half-whole diminished starting from B for bars 17 and 18 to cover the IV7 going into the #IV diminished.
Everybody got that? This will be on the exam.
The truth is, nothing he says there is that complicated, but it almost certainly is not what he is thinking when he is improvising. And I had to laugh when he introduced some of his "secret" scales later in the book.
For example, his "Charged Up C Mixo-Pentatonic b5." So, what is this deadly weapon? It's pretty much just the minor pentatonic scale with every note between the flat third and the 5th included. It's a 9 note scale. Put another way, it's every note there is is except for 3. A better way to say it would have been to point out what most jazz players know instinctively - in the minor pentatonic scale, there is a lot of tension and ambiguity between the third and the fifth. When they play, they play with this a lot. I guarantee nobody is thinking "Yes! Now for the Charged Up C Mixo-Pentatonic b5 run!"
The way it really works is summed up well in another, much better book I obtained on jazz guitar:
Learning to improvise can be a confusing experience. You are asked to deal with a large number of seemingly unrelated musical entities, then fashion them into coherent solos. On top of this, you hear the great players spin long, complex melodic lines that sounds as if they were created from the 17th mode of the cryptophyrgian harmonic major scale. They aren't. These lines are actually made up of small, simple, easily identified structures which the players have combined in interesting ways.
Great improvisers are communicating, talking through the music. And the way we learn to speak a language is to listen, imitate, learn basic phrases that we can combine to express whatever it is we want to talk about.
It's pretty humbling to realize that I can basically only say "Da Da" at the moment.