I would think that the technique of adult returners would not be a particular problem, unless not learnt properly to start with. In which case you become a beginner. Otherwise the 3 p's should do it - practice, patience and perseverance.
That kind of smacks of the old joke of how do you get to Carnegie hall - practice. While true, it doesn't really help answer what to practice or how.
KevinM, have you tried fast-slow practice? Everyone in this forum is quick to suggest slow practice, which of course is fantastic advice, but I think the other half of the equation is full speed practice.
"But I can't play full speed dummy!" you protest. Sure you can. Take a run of 3-4 notes. Play only notes 1-2 at tempo. If these are thirds (2 notes per beat) maybe just play the top note or bottom note. You should be able to play it at some absurd speed - 400bpm/16ths, for example. But play it at tempo, with the right dynamics and articulation (staccato/legato/detached/whatever). Should be pretty easy. Do it a few times. Then play only notes 2-3. Same thing - at tempo, correct phrasing. Then notes 3-4. Now, start chaining. 1-2-3. 2-3-4. Always at tempo. Always listening for evenness (or correct uneveness if the rhythm is wonky). I play a lot of polyphonic stuff, so after a few minutes I'll switch to the other hand and then practice what I need to with that hand.
I like to do these in very short segments. No more than 5 minutes at a time, preferably less. I find that as soon as my neurons start building it into muscle memory I start to falter and slow down or make a mistake. That is the STRONG signal to stop. Not so much because you don't want to practice mistakes (you don't), but because your brain is rewiring stuff. Let it do its thing. You learn this sort of thing more when you are resting then when you are playing.
Go back a bit later and start it again. Sometimes it will seem like you have gone backwards, but that is just your brain in the middle of relearning what to do, and at the moment there are some conflicting instructions. Just do what you can, then stop, let your brain do it's thing while you are not playing.
I think if you pay a lot of attention to exactly how your learning process is going you'll pretty quickly figure out an efficient path towards learning this stuff. Basically any 2 note chain should be nearly effortless, so you need to sense what is going wrong when you have notes on either side of that phase that slows you down or is 'difficult'. It'll be thing like subtle issues of hand/wrist position, unequal finger lengths requiring slightly uneven firing to get even hammer releases, and so on. Tiny changes in alignment often drastically changes the difficulty level for me.
I wrote this as if you only would do it with one hand practice, but of course you need to do with with both hands together as well. At every point it is a matter of trying a very, very short segment, seeing if anything is going wrong, and if not, then practicing segments overlapping on each side, then putting them together, seeing the problems, isolating, fixing, returning to the longer run to ensure it is all working together.
Anyway, it's how i try to practice - either extremely slow or at full tempo, not weeks and weeks of slowly cranking the metronome up a click. I don't always succeed at that, but when I do it does really seem to be an entirely different and much more successful kind of practice.
Graham Fitch has several videos on youtube where he goes into this sort of practice in depth, with examples. There's all kinds of ways to vary the practice that he goes into, and that seems pointless for me to type out when he is a teacher and I am not.