When you highlight your hair, you are actually removing pigment from the hair, which is why it dries your hair out. When you reverse that process, you are "filling" color back into the hair, which is going to make it feel fuller and healthier. This is also why darker hair always gets so much more shine than blonde hair gets. Going darker doesn't mean you have to go back to your root color all over your head and become a brunette. Darker just means any color that is darker than what you currently have.
I recommend adding dimension back into your hair by choosing a few different tones. Consult with your stylist, check out the color swatch book and choose a few different blonde and light brown tones. Add these back into your hair by foiling. This is going to make your hair look and feel much healthier and more natural. Be a bit daring and add in some warm or golden tones to really bring some fall into your hair color. Foiling is going to allow your hairstylist the ability to put the colors you have chosen together right where you want them throughout your hair. Adding two or three colors to your hair is going to give you a much more natural appearance. A person who has hair that is completely natural and has never been colored does not have just one color in her hair. Natural color has more than one tone!
I also recommend using a glaze (a semi- or demi-permanent-hair-color) on all of the hair that is not put into foils. This is just going to be like a conditioning treatment and leave your entire head of hair looking new, fresh, shiny and fabulous!
The changing of the seasons is always a good time to change your hair and your look. Even if it is something subtle and simple, think about toning down that summer blonde, darkening your already brunette hair or just going red! Whatever you do, consult with your stylist and remember that it is just hair color. You are not marrying it, and it always can be changed!