Watch the The Whole Fretboard: Mixolydian Mode online guitar lesson by Robbie Calvo from Power Play: Dominant
Improvise using pace, dynamics and crescendos over the 4 Mixolydian key changes. Remember also to resolve your phrases to sweet note chord tones, using the arpeggios if you have those in your chops toolkit.
You have the entire fretboard now to express yourself. Try phrasing simple ideas that build to a crescendo in each of the keys. Once you have completed a full track (4 sequences) take a break, get up and move around, stretch and clear your mind. These exercises are very intense so make sure you don't over do it.
The other 2 backing tracks will also serve as track to play your Mixolydian ideas over, so when you get bored with this track try using one of the other tracks. This will also give you 4 new keys for each track.