The best way how to thicken the sauce in case it is too runny is to let it reduce on a medium simmer. Longer cooking doesn't have a negative effect on the sauce, it will enhance the flavours. Don't worry to let it boil over 30 minutes if necessary. If you don't have time and your family is ready to eat you for their dinner you can thicken tomato sauce with either flour or corn starch (corn flour). If you don't know how much of the sauce liquid you need to thicken I recommend starting with one heaped tablespoon of flour or corn starch. Mix a tablespoon of thickener with a small amount of water to make it runny and pour the runny mixture into the sauce. Stir continuously. Boil the sauce for 1 - 2 minutes to see how thick the sauce is. Repeat the process if necessary. One thing that can be tricky is to make the thickening mixture without lumps. I would like to share with you a trick my granny told me that prevents lumps in it. Always mix flour with hot (not boiling) water and corn starch with cold water. It's a simple but clever solution that works.