It is a better idea to use your leg muscles to carry the pack directly than to use them to cycle while wearing a pack. This is a closer simulation to what you will be doing.

Another good thought would be to include carrying your pack on actual trails, since the rocky, root-strewn, uneven surface of most trails require working the leg and ankle muscles that provide lateral stability. Stairs won't work those muscles. It is a truism among hikers that the best conditioning for hiking is to hike.