You're new to the school and new to teaching. You're not familiar with A: schemes of work and existing resources, so you're trying to do everything from scratch. You want to make a good impression on your colleagues, so you're probably trying too hard to be self-reliant. Established teachers have routines, banks of resources, and tried and tested approaches to almost every classroom situation. Talk to them, they'll be happy to share, and your planning will become much more focused.
Staying behind for at least an hour and, as a result, being able to take Friday night and Saturday off sounds quite efficient to me. Many teachers begin their working day in school at least an hour before lessons start, and would be surprised if they were able to leave before 5pm without taking work home. In that context, it doesn't sound as if it's taking you too long to prepare. However, Parkinson's law also applies to teaching, and your work will expand to fill the time you allow it to.
Your preparation will make more sense if you see it in context. What are your pupils supposed to be learning this term? Take an overview from your schemes of work so that you have a broad idea of where you're going, but don't plan in detail for more than a week ahead. Then, when you sit down after school each afternoon you will have a clearer idea of what needs to be done for the next day.
Be careful about working through breaks and lunchtimes - it's important to have that informal time with your fellow teachers as that's where you pick up their hints and tips.
It's good to want a clear demarcation between home and school, but don't become resentful when work intrudes on "your" time. Becoming a professional means focusing on the task first.