Slow your roll, eager beaver. You don’t need to overcrowd the gym on January 2nd. Instead, baseline your normal activity and abilities for a week. What is your one-mile run? What’s your squat max? How many hours a day are you active? How much are you sleeping? What are you eating?
Personally, I log everything on a Google spreadsheet, but all of the fitness trackers have their own daily logging methods.
Now, make one (one!) significant improvement and see what happens over the next three weeks. Cut out grains (yes, all grains). Set a consistent bedtime. For muscle gain, try bulking up with 0.8 gram of (healthy) protein per pound of body fat per day.
I find that setting experimental results helps me commit, because I know that a single misstep can screw up weeks of effort. If your experiment works, great! If not, there’s something wrong and you need to re-evaluate. But since you only changed one thing, you know what works and what doesn’t.
Avoid broad changes, like “eating healthy.” Instead, try exchanging one of your meals for a salad, but with the same number of calories (olive oil or avocado is a great way to pack in healthy fats).
Health is a marathon, not a sprint.