I was sick, tired and had lost myself until I began lifting weights at 71
Find an exercise/activity that you enjoy or makes you feel good.  Doesn't matter if it is weight lifting, road cycling, or dancing.  Once you find it, just keep doing it.If you get bored, try something else for a while.  If that doesn't work, switch back.Since no two people are the same, no exercise plan works for everyone.  For me, weight lifting and technical mountain biking get me going and make me feel great.  Tennis used to be a thing I lived for.  Running and road cycling is a agony for me.  Dancing is great.  But I have friends who love to cycle 100 miles in a straight line.  I guess there's some mental thing happening, but I just cannot do that.An activity may really wear you out, but if you feel more energized the next day, then it's a good one for you.  And if you start craving that feeling, then you know you found a good one.Even within one type of exercise, there are many different approaches (which work better or worse depending on the person).  Low rep, high weight is my thing.  High rep, low weight (like road cycling) is a muscle burn and mind burn killer.  So if you try something and it's not for you, make sure you try a variation on that before you decide the entire activity is bad.Diet is important, but it can be easier to follow a good diet when you already feel better from the exercise.  And you may find that you have more discipline after a good workout.  On the contrary, a day which sucks and you can't get up and go to the gym is a day you're also likely to order pizza and beer for dinner.