Many phones can automatically log your activity, or lack thereof. I used to use Samsung Health.

Fat people aren't fat because they don't move though, they're fat because they eat too much.

Take me as an example, last I checked I have a 2200 calorie sedentary BMR. An hour long walk is only going to burn 100 calories and wouldn't even burn off a single soda with dinner, let alone if you drink them all day.

The easier option rather than try to ambulate your blob down the street, sweating, wheezing, and murdering your knees, is not to drink soda.

Soda is a big one, that's why I keep mentioning it. Switching to tea, coffee, or bestest, water, is all a lot of overweight people need to do.

But there's a few other calorie bombs people need to be really careful to enjoy in moderation that aren't as obvious as say, funnel cake or ice cream. Such as bread, cheese, booze, granola/cereal, and peanut butter.