Early 50's sprinter here - was once a middle / distance guy and have spent far too long on these boards over the years.
I've been back training a year or so after a long layoff. Lots of useful stuff posted above.
First thing I did (first session) was injure a hamstring and I realised that I had to build some basic fitness as I had none at all. I started jogging shortish distances and added some basic drills (learnt off another, very accomplished masters athlete) and slowly started to feel more athletic. Unfortunately I suffered further problems and had to restrict my workouts to drills and some not very intense workouts. This lasted several months.
I managed to 'race' 100m through the summer, slowly introducing some faster work in training but still spending a lot of time doing form drills. I improved in every race apart from the second one (caution over an earlier pull) and even managed a few 200s. A bit sobering that, by end of season, I had only just managed to run 200m at my 20y/o's 800m pace.
Like the majority of the northern hemisphere's athletics community, I am now into winter training. The entertaining bit here (and through the summer) has been trying to work out how to structure a training program. I have spent a long time reading the various online resources, textbooks, watching YouTube channels and browsing r/sprinting. I can't find anything structured that is masters specific. Fortunately I quite enjoy this.
Early winter I scheduled in a lot of tempo work. I have some health issues that cause me to limit the distances I run, so tempo distances were short but with a reasonable number of efforts. I found these be really easy to do I suspect due to decades of earlier distance training / running.
Tempos have been a good way of practicing form and building leg "resilience" without overtaxing the system.
I know from experience that I have to be careful scheduling really intense work as I don't always recover quickly. This is improving but is a lot to do with why I don't just train with the club's sprint group. Accelerations don't cause much grief now but high intensity flying 20's (for example) can keep me sore for longer than the wait for the next track session.
I have introduced some easy plyos to my training and try to include some in every track session. These appear to be having a massive effect on my basic speed (I'm not timing but the improvements are pretty obvious)
One thing the distance runner in me has had to let go of is the urge to keep increasing the number of efforts. More is often less in sprinting, it would appear. I might be compensating by doing loads of drills in warmup but I need a good 35-40 mins before attempting anything quick otherwise I hurt myself. I tell myself that more drills can't hurt too much.
I'm probably not best placed to offer any advice but I do suggest that all changes to training are gradual, as is the introduction of any new methods.