Yes, you are correct. It has the meaning to be energized. Why is juices plural? Your guess is as good as mine. The body is full of fluids.

Don't know why "juices" is plural but when referring to bodily fluids, whether literally or metaphorically, it is usually in the plural form. For example, we are often told to cook chicken until the juices run clear. Another example of metaphorical usage: "get you creative juices flowing", which means to exercise your creative ability.

Juice is a non-count like the word milk or water. However, in this context we like to think of the mind or the body as having lots of different kinds of juice flowing. You are right to say it should be juice but this this is abstract thinking about getting enthusiastic. It sounds like a lot more different thoughts are working if we imply lots of different juices flowing.

how can it be plural it cant