I'm doing like Mace has said, with a slight embellishment;
I start at the 8th fret low E string and play chromatic up to the 11th. Keeping each note fretted as I play them until all 4 notes are played. The key is to keep all fingers close to the neck as you go. The point is your training your standby fingers to be close and ready, not curled up under the neck or pointing away. Try to keep your thumb central ish on the back of the neck. But most importantly go slow, very slow. Your training your fingers but your also exercising them and building strength. Slow is vital. Work your way up the strings, E then A then D etc and then all the way back again. The slower the better. Then after 1 cycle of this I do the same again but move the little finger up to the 12th fret so I play 8, 9, 10 and 12 on each string, slowly across all strings. Trying to keep all fingers close and ready. Remember to keep all notes fretted until you change up or down strings. You start to feel it by the time you get to the high e string. After one cycle of this i then play 7, 9, 10 and 12. Again across all strings and back again. Absolutely critical is you remember to go SLOW and keep all notes fretted as you go. Then move the whole thing down a fret and start again, if you can get this down towards the 3rd fret your really feeling it. Dont go any further than when it gets painful, you don't force it, after time your fingers will strengthen and stretches will become easier. It's a marathon not a sprint.
One other exercise I'm doing is this;
Again keeping thumb behind, all fingers close by strings and ready, 1st finger 1st fret hammer on 2nd finger 2nd fret from high E to low E and back again. Slowly. Then 1st finger 1st fret hammer on 3rd finger 3rd fret high E to low E and back. 1st finger 1st fret hammer on 4th finger 4th fret repeat cycle. Then start with the 2nd finger 2nd fret hammer on 3rd finger repeat cycle, then 2nd finger 2nd fret hammer on 4th finger 4th fret and repeat cycle. Finally 3rd finger 3rd fret hammer on 4th finger 4th fret and repeat cycle.
Then repeat with pulloffs.
The point is I'm exercising my fretting hand and training the muscle memory at the same time, after a week and a half I'm already seeing a marked improvement in both tone and stamina. It really does help your tone, I'm fretting notes much more cleanly which is in turn helping everything else.
Once I'd had this word with myself and deflated my ego, I realised I'm probably carrying so many bad habits after 20 years of just playing and not learning I really needed to get back to the start and unlearn those bad habits. Next up will be re-learning the scales and modes and actually learning to play lead, not just tabs from songs. But and it's the biggest but, I have to get the fundamentals right first. And those fundamentals are good proper acurate and consistant single note and chord fretting technique and accurate consistent picking technique. Its not gonna be easy and its not going to happen over night. It takes years to really become great at anything but it will be worth it.