Welcome to SoCalHiker, Mick!
I love backpacking, and the Rae Lakes Loop is a beautiful (and very popular) route. Since you’re looking at doing this next summer, I’d say you have plenty of time to prepare.
I would recommend that you consider a couple shorter trips closer to home to get familiar with your gear — what works, what you want to change — before you head out for five days on the trail. You should also be confident that you are physically prepared to hike the mileage and vertical gain with a backpack.
One of my favorite local “shake-down” trips is the Trans-Catalina Trail. Really nice in the spring!