Warming up the voice is the number one most important thing you can do before you start singing.  It's equivalent to stretching your muscles right before a workout.  Your vocal muscles become stretchy and ready to sing more difficult runs during your warm-ups.  Do 15-30 minutes of warm-ups to prepare for singing. And sing at least 2 whole steps higher than the highest note you plan to sing.  

In order to build up proper vocal endurance, you should practice the warm-ups in Own Your Voice for at least 4-6 weeks.  You may even start to notice a difference right away.  You'd be surprised what warming up the voice can help it accomplish.  To build vocal endurance you can count on in a live show or recording studio, I'd say give yourself at least 4-6 weeks to build up your vocal endurance.  They key is consistency.  Do the warm-ups every day and you will feel and hear results before you know it. 

I recommend you do at least 15-30 minutes of warm-ups per day as well as work on singing songs of your choice for another 30-45 minutes.  Depending on how your voice feels after 1 whole hour of singing, you can decide to stop or keep singing.  The voice has about 1 whole hour of singing before it starts to get tired. 

That's not to say you can't sing longer. I've done 4 hour shows, and my voice is very tired by the end of those shows, but it is possible to sing longer than 1 whole hour.  The key is I am not singing every second of those 4 hours.  There is a lot of instrumental music to cover ground for me during those longer sets of music.  

Just remember, when your voice starts to feel tired, stop singing before you do any damage. 

Hitting high notes depends on a few things. 1. In order to increase your higher range, you must over time increase the highest note you are singing. So, for example, if on day one you can only hit a high F comfortably, in one week, go to the high F#, then one week after that try the high G, and so on and so forth. 

You'll have to go at your own pace as well.  Just make sure you're using the proper technique and you can increase your high range by many steps.  Just remember, everyone is different and everyone has a limit too.  It's important to understand your own personal range and what's physically possible for you.  As we know, just like body shapes, voices also come in all shapes and sizes, so this will naturally determine our personal vocal ranges. 

If you are curious about your vocal range, book a private one-on-one coaching session with Pamela Parker and she can help you figure out where you're currently at with that.  She can also work with you to increase your higher range and make it more comfortable to sing up there.