Do you have a smart phone? If so there are several metronome apps available. I am 54, and if I can offer some advice based on what has worked for me:
I use a metronome app and do some basic "pedaling" on open strings at the beginning of every practice session. For example - I start at 120 BPM, and I play 16th notes for a total of 10 minutes, 2.5 minutes for each string (I play 4 string). Yes it can be boring/monotonous. When I finish at 120 BPM, I bump the tempo up to 130 BPM and do the same thing, except 4 minutes (1 minute per string, still playing 16ths). Finally, I do the same again at 140 BPM - 16th notes at 1 minute per string. I've been doing this for more than 5 years at approximately 4 to 5 times per week, and I didn't begin at 120, 130, and 140 BPM - I had to build up to it - but it has helped immensely. And the key is the constant repetition. Not only does it help with speed and stamina, but also with consistency. When I play lengthy passages of 8th/16th notes, the notes are more consistent.
That is my routine for right hand (plucking hand), and it takes less than 20 minutes. My routine for the fretting hand is longer/more intense, it takes about 30 minutes, and it is made up of permutations and other repetitive pattern exercises.