You should probably walk outside if at all possible. It's much more fun than a treadmill and probably burns more calories, climbing up hill, going down, etc. If you're not good with people, not too many people are on hiking trails, so if you can get on them, it's fun.
As far as situps, people misunderstand situps. Are you doing 500 situps a day or 500 crunches? Either way, it's probably too much and rather pointless. You can't "trim fat" around various bodyparts by just working the muscles, no matter how many reps. Fat is lost all over the body when you lose fat, though sometimes because hormones, your body will store fat in different spots more. If you want to see your abs/flatten your stomach, you'll have to lose more bodyfat. The situps will strengthen the abs somewhat or give them more endurance, but you will not see your abs regardless of how muscular they are if you have fat covering them.
For diet, it again, depends on bodyweight and if you wish to lose weight/lose fat. I'd say 2000 is probably good for your activity level, considering walking is pretty low intensity. The only way to know for sure is run your bodyweight through a calorie counter type thing, and if you wanna lose weight, do a deficit of 500-1000 cals a day. Of course, it also helps to know your bodyfat levels. Some people are skinny and high bodyfat, and other people are technically overweight and have low bodyfat and will weigh the same. Also, it's easier to calculate calories needed if you have your lean mass levels and bodyfat levels known. BTW, generally 10% bodyfat will make you look "ripped" but imo, most people will consider you "skinny" at under 20% bodyfat.
Good luck!