So how do you develop this flexibility? Of course, practicing pedal scales and arpeggios in all keys over one and two octaves. Also today's exercise in sight-reading will help you do that.
That's Etude No. 1 from 12 Etudes for Solo Pedals (ca. 1865) by the Romantic French composer Charles-Valentine Alkan (1813-1888). Besides the trills, in this etude you will find ascending and descending arpeggio figures. For the sake of safety, please wear organ shoes with heels while playing it. Take an extremely slow tempo and aim for the nice legato touch.
Don't forget to change position in extreme edges of the pedal board by pushing off the opposite foot. Curiously, the pedal part in the fourth line descends to low B. On modern organs, play measures 2 and 3 of this line one octave higher.