• Your body is now "out of warranty". When it breaks in a significant way, it may never get fully better. Don't do stupid stuff that will break it in those ways.
• If you haven't figured out yet that you can't drink, party, or skip sleep like you did earlier in life, learn it quick. Each of these will have a much larger impact on you.
• If you don't have your retirement savings on track now, you have a VERY VERY short window to correct that. Its going to be much harder now to put the money aside than it would have earlier in life, but you're in the prime earning years of your life. I would think you will earn more now in the next 10-12 years or so than you will/would have in any 10-12 years of your life.
• Hopefully you've learned by now to be comfortable with yourself in your own skin. This means not doing things/buying things to impress other people. You are who you are. This doesn't mean stop learning or improving yourself, but nobody is going to be impressed with a 40+ year old driving an amazing car. It doesn't matter how amazing the car is.
• Support younger people. You didn't get where you are on your own. You had help, advice, and your mistakes were forgiven by those older than you because of your youth. Now its your turn to do that for the younger people. You grew up seeing heroes around you. You are now the hero in a younger person's eyes. Hold that duty sacred. Be their hero in how you act.
• Embrace change! No, things aren't like they used to be. No, they shouldn't necessarily go back to those things just to make you comfortable. Its not our world anymore, it belongs to the younger generations. Understand your ways could be out-of-date or backwards. Your old ways worked for you in that old world. That old world is gone. Be part of the new. You'll have to do this two or three more times before you die if you live into your 80s or 90s.
• Stay fit. That doesn't mean body builder or supermodel. It means be moderately active so that picking something up doesn't cause a strain injury that will take you 6 months to a year to recover from. You shouldn't be out of breath walking up stairs. If you are, make changes. You're likely already seeing the difference in your peers with those that stayed fit vs those that didn't. Be part of the first group, not the second. If you are in the second group, you can change to the first by taking care of your body and eating properly.
• Make a will. You will die and there is nothing you can do to stop it. Dying without having a will makes some of your assets evaporate in probate and may mean your money/assets go to people you may not want.
• Go to the goddamn doctor for wellness checkups! You're entering the time of life where conditions found early are treatable. Don't dying/become disabled because you were too lazy or afraid to go. Yes, some of the things are going to be uncomfortable, but I can tell you its much more comfortable than debilitating pain or death.

You might look at the list above and think it negative for you now. Its not at all. This is the prize for living to this age. You likely have some friend or family that died before 40. You are here to see this part of your life. Make sure you're still here to see 50, 60, 70, with as much of your body and mind as you can.

Good luck!