Istinja means ritually cleaning your private parts after using the toilet. It is not obligatory to do Istinja immediately  but only for performing Prayers (Salah) or Tawaf around the Ka'bah as one of the conditions of Salah to be ritually pure (Taahir) in your body and clothes. If you use the toilet and did not do Istinja, it is permissible to wait till the time when you want to perform Salah and do Istinja before Salah. Even your Wudhu is valid before doing Istinja because the parts of Wudhu only should be Taahir while performing Wudhu and not all the body. All body Must be Taahir for Salah and Tawaf.

Wassalam.