So it's now undeniable that I've had serious and untreated ADHD my whole life. I'm in my mid 30s what would being medicated realistically feel like for me?

1. ADHD medication isn't a magic cure that makes all of your ADHD symptoms vanish, it just makes it easier to deal with them.
2. Stimulant meds will vary in effect depending on the person. Dr Russell Barkeley, a recognised global expert in ADHD, puts the efficacy of stimulant meds at about 40% for the amphetamine-based meds and about 40% for methylphenidate, so about 80% of ADHDers respond well to stimulant meds. For me, I barely respond to methylphenidate. For others methylphenidate is a game changer. Go into it open-minded but without any particular expectations.
3. It's a good idea to go easy on caffeine, especially in the early days of stimulant medication.
4. Generally ADHDers find that stimulants reduce their anxiety levels.
5. Starting on stimulant meds usually comes with mild euphoria. Enjoy it while it lasts, don't go chasing the euphoria though.
6. If you have a history of significant trauma then often stimulant meds give your brain enough breathing room to start processing that old trauma, like it or not. This can get a bit hectic for people who have a lot of unresolved trauma so if that's you then make sure that you have the right supports in place.
7. Stimulant meds will change how you manage things and how aware you are of stuff in general. This is almost certainly going to change your close relationships, sometimes for the better and sometimes for the worse. At its best you're going to be a better friend and partner because you'll be able to attend to other people's needs and be more consistent with coming through for them. At its "worst", you might come to the realisation that you are being taken advantage of or you are being abused. Just be aware of this.
8. If you are also autistic, auDHDers find that their autistic traits become much more apparent and that they find it more difficult to mask when taking stimulant meds.
9. You will have a lifetime of unhelpful habits and beliefs that you have accumulated over your life up until this point which are unsuitable or downright counterproductive for an ADHDer. Meds make the doing side of things easier but the habits and beliefs will have a much longer lag time and you'll have to work on those, ideally with someone who understands ADHD well.
10. ADHD meds will give you the ability to focus and sustain that focus, which is wonderful. They don't instantly provide you with the self-discipline to focus on the most urgent priorities though so you might find yourself furiously researching some historical event or reorganising your kitchen when you really need to be packing your bags for your flight.
11. Often ADHD symptoms and an individual's psychological response to them can mimic depression and treatment-resistant depression. You might find that if you're prone to depression that some of this is alleviated by the meds.
12. Overall it's better to stay consistent with taking ADHD meds even on off-days or weekends because it will make it much easier to establish different habits and a new attitude towards things, however it's also okay to take medication vacations particularly in the early days as you're adjusting. My general advice would be to avoid taking medication vacations more than once a fortnight, except where there's good reason (e.g. you're sick, you're having difficulty with side effects, you need to sleep in).
13. You will be able to push through without food, breaks, or sleep. Don't do that. Schedule in breaks, make sure that you are eating well and getting enough sleep or you will crash and burn.
14. You may find that you have a significantly reduced appetite. Try to have a good, high-protein breakfast in the morning to carry you through. Protein shakes and smoothies are ideal.
15. Depending on what you get prescribed, you might benefit from a prescription that is somewhat higher than your baseline level of need - stress, lack of sleep, poor eating, unusually high demands, lack of physical exercise, and similar things can all make the meds feel less effective or ineffective so having some wiggle room to adjust upwards on the meds based on your needs is Ideal.
16. If you menstruate then you go through a hormonal cycle where you will naturally have less dopamine in your nervous system. ADHDers who menstruate very often suffer from PMDD. ADHD meds can often help relieve symptoms of PMDD however the meds often feel less effective or ineffective so it's ideal if you are able to increase your medication dosage around your period.

I can talk more about managing insomnia and increased anxiety on ADHD meds as well as second-line ADHD meds if you need the info but that's likely something for down the track, if you encounter these things.

Edit to mention that I'm also pretty knowledgeable about the less commonly prescribed ADHD meds and all of that stuff as well so if you have any questions about ADHD (not to mention autism, mental health, and medications more generally) then let me know.