The usual Stepper library is blocking, that is, it will not continue until it has given the number of steps.
The solution is a loop that closely resembles the "BlinkwithoutDelay" example. Instead of togging pin13 you call step(1) whenever the "millis()-timer" differnece exceeds the timing threshold. In the loop you also do a an analog read on every pass and calculate an updated value for the timing threshold.
Do not call Serial.print on every pass. This takes too much time. Only call it every 1000 millisec (which means you have another section with another timer that triggers once a second - have a look at Blink Two LED without delay)
Enjoy