I've also always had weak knees and I hate running and high-impact footwork because of it. I waitressed last summer and spent the better part of eight or nine hours on my feet. Here's my experience.
If you don't have a specific diagnosed problem that requires knee braces, don't use them long-term. They're meant to give muscles and joints a rest so that they can heal, and if you use them when you aren't injured they'll actually end up weakening muscles that need to get stronger to solve the problem. Even when I used one for a few weeks after dislocating a kneecap in karate class, that happened. My knee was weaker afterwards because the brace stopped me from using certain muscles in addition to the tendons that were healing.
One really important thing to do right now, to avoid actually doing long-term damage to your knees, feet, hips and back, is to wear proper footwear. Don't cheap out on shoes. Get shoes that are designed for standing and walking all day. There are stores that specialize in work shoes for people who stand all day, like cashiers and nurses. I wore Doc Martens all that summer and while I had muscle aches from fatigue at the end of the day, I didn't have any tendon or joint damage.
For the first little while you'll be sore, just like when you start any new exercises that you aren't used to. Take hot baths when you get home, and use something like Icy Hot, Tiger Balm, or Ben Gay if you have actual muscle aches that keep you from sleeping or relaxing. (Try the topical creams and hot baths before you resort to painkiller or anti-inflammatory pills.) Before long you won't have to do that anymore because the muscles will be adapting to the workload.
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