Now at 70 and retired with plenty of time. I am doing a fair amount of speed building. I have always been taught that the faster you are, the easier it is to improvise.
So about six months ago I started. I couldn't do major scales, sixteenth notes at 80 BPM. Now I can do scales at around 100 BPM. I can do bursts of 4 to 8 notes at 120 BPM. I can do eighth note triplets at 120.
I use the GT metronome. Using the auto increase BPM I increase 4 BPM every 4 measures. Once I could do 80 BPM I would do scales and modes from 80 to 100. Now I am working from 120 to 140.
I think the biggest obstacle to progress is I am also continuing to learn new songs, usually from the 50's to 70's. (Too many songs not enough time.)
Another obstacle is I have been doing this alone for 50 years. Limited opportunities to play with others is a serious impediment to progress.
At our age we need to keep moving these fingers.