Well I'd stop letting people in their 50s overhearing me say 35 is 'midlife', for a start! Yeah, logically it is, but LA LA LA. I am reminding myself of my mum in her late seventies rejecting a clothes catalogue because everything was 'awfully middle-aged'.
But really, I think it's all the unglamorous, tedious shit you know already that you'll probably be pleased you did: as Baz L says, stick with the SPF. Try not to get very fat or very thin and don't do 'diets'; eat your veggies, ditch ultra-processed food as far as you can, but definitely do deliberately enjoy whatever gives you pleasure wholeheartedly, whenever you get a chance.
Don't stay in a romantic relationship which makes you miserable: sort it out or separate. If you are determined to martyr yourself no-one can stop you. All this stuff is easy to say and hard to do, in my experience and hindsight is a bitch.
Keep in touch with your siblings and/or cousins etc and friends from school if you can. They will be there and will remember who you really are when grandparents and parents and uncles and aunts are gone.
Find out how to enjoy extended time alone. In the long run, our brains make it seem that time passes more slowly when we keep making them learn new things, essentially giving us what seems like a longer life, so pack in as many 'new' experiences as you can each day or week or month, especially ones that challenge you a bit. Sometimes we don't choose or like the challenges, but at least they keep us learning, which keeps us young and mentally agile.
Now pass me my pipe and slippers, I'm a bloody sage for the age, quoth she.