Regarding “diet” or nutrition: I used to think you could get by with just habits, that is, “good eating habits”, like refraining from eating “dirty / deadly dozen” type foods. I found that over the years, good habits are almost impossible to keep up without some sort of accountability. If this is something you are dealing with, the very least you can do is keep a food log. Just write down everything you eat in a little book, on paper. One page a day. Or in Notepad++, or any simple text editor. I like regular notepad.exe because you press F5 and you get the current time and date. Using myfitnesspal worked well for me for a couple of years but it was a gigantic pain in the ass to use. I suppose it was semi-decent for tracking macros and total calories. But that shit got hacked, so forget about it.
Also, my opinion: don’t track calories burned doing exercise. You almost have no idea how many calories you burn when you perform an activity unless you hook yourself up with lab equipment. You overestimate calories burned, then you eat a ton of excess food to compensate. Just like you’ve been doing all those years that got you soft in the first place. Just do your activities, enjoy them, log what you actually did, and reflect on how you felt doing them. I like how GMB deals with RPE.