I suppose this is pretty subjective and depends on your level of knowledge on the issues, but I feel that healthy living is more or less a solved problem from a "what to do" standpoint. The main problem is breaking people of unhealthy lifestyle habits while educating them on healthier habits (see books like Power of Habit for habit formation and modification).

I don't have links handy (edit: apparently I have a few) but just from reading over the years, it seems that the highest impact things you can do (that you have direct control over) to avoid the diseases of aging (both physical and mental degeneration, as well as the usual lifestyle diseases like diabetes and heart disease) while increasing or at least maintaining one's quality of life include (1) regular strength training, (2) endurance training, (3) avoiding processed foods and eating significantly more vegetables, (4) minimizing visceral body fat levels, and (5) always be learning something new (particularly relevant for lowering risks for cognitive degeneration).

For more specific and actionable advice:

1) Every able-bodied adult should invest anywhere from 6 months to 2 years of dedicated strength training to build muscle mass and strength. The benefits are numerous and well-documented (e.g., https://en.wikipedia.org/wiki/Strength_training, http://startingstrength.com/article/science-medicine/barbell...). A somewhat arbitrary floor recommendation would be to maintain a level of strength that allows you to deadlift twice your bodyweight and accomplish 20 pull ups in a row. This is a non-trivial level of strength that requires training, but is achievable for most people with intelligent (but not necessarily complicated) training. Any competent strength coach can take you there. Reading the book Starting Strength by Rippetoe would be my recommendation of where to start.

2) The benefits of endurance training are similarly well-documented (e.g., https://fivethirtyeight.com/features/the-5k-not-the-marathon...). The actual modality doesn't matter, but the only thing I know anything about is running so I'll stick with that. Rowing or swimming or hiking would also be great. A non-trivial goal to try and hit would be a level of fitness that allows you to run a 5k in ~20 minutes or less. If you search r/running, you'll see that this is surprisingly achievable as well. You simply have to work your way up to anywhere from 30-50 miles per week of running at a conversational pace. A good place to start would be Couch to 5K followed by Bridge to 10K. Then follow the 10% rule for weekly increases in mileage. The Running Order of Operations linked in the r/running sidebar has great recommendations. (Other tips would be to make sure you're running slow enough; the conversational pace tip is one metric, but another is to make sure your heart beat stays below 180-[your_age]. Running the first few months with a metronome app going in the background at 180 bpm and trying to sync your steps to it and trying to minimize how loudly your steps are will also minimize the impact forces and help you run more efficiently.)

3) Nutrition is filled with a bunch of contradictory advice, but there are some pieces of advice I like that seem unlikely to change in the future. In terms of what to eat, Michael Pollan's "Eat food, not too much, mostly plants", is a great place to start. Another person I encourage people to look up is Rhonda Patrick. She's a researcher who summarizes a lot of research about nutrition and its interaction with physiology and genetics. If you follow Pollan's advice of avoiding processed foods and eating more plants, you'll be much better off. But if you know the current state of your nutrition (e.g., based on what you're eating, how your weight is changing from week to week, blood panels to check for nutritional deficiencies, etc.), then Patrick's advice to get at least 100% of your RDA of various micronutrients will serve as a great framework for how to modify your diet further. Sites like Cronometer.com or NutritionData can compare your current diet and show you the likely distribution of your nutrient intake based on your diet and allow you to model what the addition or subtraction of various foods would do to that distribution. For example, you might find that you're regularly deficient in choline, and mucking around on Cronometer might show you that the addition of 2-4 eggs to your daily diet would resolve that.(Other really useful resources include Wikipedia and examine.com)

4) The strength and endurance recommendations above are made much simpler if you aren't overweight. But instead of chasing abs (not that there's anything wrong with that), I'd recommend trying to get leaner until you've minimized the amount of visceral body fat you have. Visceral body fat is something that has a lot of evidence suggesting it's connected to several disease states (http://www.health.harvard.edu/staying-healthy/abdominal-fat-...), so minimizing it gives you a non-vain metric and motivation for getting leaner that should increase your health along a variety of objective metrics. Now you can't spot reduce fat, so the only way to reduce visceral body fat is to lose body fat in general (genetics determines at what point in your fat loss journey your body decides to dip into your visceral body fat stores for energy). The sidebar in r/fitness and r/loseit have more information on this improving body composition, but the main points are to maintain a caloric deficit (https://examine.com/nutrition/what-should-i-eat-for-weight-l...) and eat enough protein (aim for 1 gram of protein per pound lean body weight; https://examine.com/nutrition/how-much-protein-do-i-need-eve...).

5) Anyone who's done any time dipping into the research on the diseases of cognitive decline (e.g., Alzheimer's) will know that, in addition to diet and exercise, one of the most important things you can do to lower one's risk is to maintain a life-long habit of learning new things. By that I don't mean trivia or a Wikipedia page per day (though there's nothing wrong with that), but the learning of things that require you to really work for improvements. I'm talking about stuff like music and playing instruments, foreign languages, pure mathematics, chess, go, art, dance, etc. You have a lifetime so you don't have to choose just one, just make sure it requires you to put in the work and struggle a bit for improvement. Coursera's Learning How to Learn course would be an excellent place to start for the "how" of this, and your own interests will probably lead you to the "what". (For languages, I'd recommend the book Fluent Forever.)

The only other major factor that seems to really contribute to long-term health is something that isn't entirely within your control, but you should still put a lot of effort into it: regular interactions with people you love like family and friends. If you don't have those, try and go out of your way to expose yourself to more people so that you can find your people.

Other things in research that have been getting more attention include things like meditation and practicing gratitude, but their benefits aren't as objectively measurable as the above, though they're probably well worth investigating and putting time in to.

Oh, and make sure you prioritize getting sufficient amounts of high quality sleep. HN has had conversations in the past about this.

In general, "use it or lose it" seems to be a universal law when it comes to one's mind and body.