If I stop my daily 10 minute running, and do extra cardio e.g sit ups instead, will I put on weight?

Sit ups aren't considered cardio, they are considered muscle toning.

You put on weight (fat) when your calorie consumption exceeds calories used by your body. If you drop your 10 min run (which at best likely used no more than 100 calories, probably less), then replace that with some other activity to burn those calories OR cut some calories from your daily diet.

No, but try to add in some aerobics too! It's very easy to do while also very good for you.