AWOL:
I'd rather you thought about it a little longer - it really isn't difficult, and it won't take long to prototype.You already change the state of the LED (on or off) at some time determined by the expression
if (millis () - previousMillis >= interval).
At the same time as you're changing the state of the LED, you could change the value of the interval that the LED remains in that state.
The interval is changed by pot, so, honestly, I have no better idea 