You shouldn't be typing at a constant / consistent speed. When you're at a section that's easy to type, speed up. When you're at a section that's a bit more tricky, slow down.
Different words fall under our fingers differently; sometimes the process of moving from one key to the next is very smooth, sometimes your fingers get in the way of each other.
As you type more you'll get a better feel for what is and isn't easy to type, and you'll gradually adjust your fingerings to make some difficult sections easier, letting you type at closer to max-speed for longer amounts of time, and thus increasing your wpm.