I installed a new sink disposal, and got to the last step before realizing the discharge pipe does reach. What do I do next?
No need to rebuild the entire assembly! It's a simple fix, you just need a couple parts from the plumbing shop. All you need is a short flanged tailpiece (see below) and a slip-joint elbow (see below).
Use the metal retainer and gasket that came with the disposer on the flanged tailpiece after you have cut it to fit. Insert the tailpiece into the elbow after it has been cut to fit... voila!
You are going to have to re-build the part outlined in yellow below. The part that comes from the second sink drain is the last part that you will replace.
The new lower section of the P-trap will need to be rotated toward the back of the cabinet to take up the gap left from the disposer to the outflow pipe. Note that the disposer may also need to be carefully rotated some to bring the outflow pipe into alignment with the vertical from the P-trap.
The last thing you will work on is the length of the horizontal section from the second sink drain to the Tee in the vertical from the P-trap.