I've been playing for quite a while, but never really learnt how to string a solo together, I've always played more like hendrix where he plays rhythm but with lead fills everywhere.

Where should I start in learning how to improv solo properly?
"I think the most important thing about music is the sense of escape." - Thom Yorke
Have you ever learned to play a full solo all the way through by any of those players?
Learn their songs, learn their licks, analyze it and get it in your head.

See what chord/chords the licks are played over and how that relate to the key.

GET THE LICK INTO YOUR HEAD. You want your ideas to be as clear as when you talk, you don' think about it. I bet you know the sounds of stuff like happy birthday in your head, you can instantly recall how it goes, that's what you have to do with everything you learn.

Use your ears.
Fusion and jazz musician, a fan of most music.

Quote by Guthrie Govan
“If you steal from one person it's theft, and if you steal from lots of people it's research”

Quote by Chick Corea
"Only play what you hear. If you don't hear anything, don't play anything."
simple answer. find a backing track on youtube and play with it.

Tough answer. Learn the solo's of those players.