Hi,
Here's a page that may have more intelligible explanations for you:
www.pjrc.com/teensy/td_libs_AccelStepper.html
Note the description of setMaxSpeed, in particular: "When controlled by setting position, the stepper will accelerate to move at this maximum speed, and decelerate as it reaches the destination." It's an automatic thing that happens with runToPosition.
If I understand this right, setMaxSpeed will set the speed at which the motor moves from step to step if you program no delay between steps. You change running speed by changing the timing of when steps are initiated.
If you want to change the speed while the motor is turning, you modify the variable passed in the set speed function: mystepper.setSpeed(stepsPerSecond), incrementing whatever variable you use for stepsPerSecond to accelerate or decrementing it to decelerate.
Remember that stepper motors were invented for positioning control. Generally in positioning you want to motor to move as quickly as possible from position A to position B. But trying to go from 0 to max speed in one step--or from max speed to zero in one step--is a problem because of inertia and is likely to make the motor skip steps and lose track of position. That's the main purpose of the acceleration and deceleration that the library provides.
Hope this is helpful.
--Michael