Before firming up the existing plan, I would recommed spending some time thinking about what you want your layout to look like. Glenn hinted at this in saying think about the scenery first. Since you have space for more than just a table top, getting your preferences reflected in your track plan will really make a difference. More than a table top also means more than a table top's investment in both time and money, so getting a plan that will satsify through the layout's "completion" is important.
You also allude to this in stating your desire to lengthen the front passing track. First, you/we need to know how long a train do you want to run. Unless you go to double track or some form of twice around continuous plan, I would recommend your goal be 2 trains 4ft long or less. Longer than that means a different type of operation and plan - but that's one of the choices you need to make. Train length will drive passing track, staging track, and yard track lengths. Train length also controls distance between towns - you don't want your train in 2 towns at the same time - and acceptable grade steepness. Most engines aren't going to pull a 4ft train in N up a 4% (probably not 3% either) grade.
Next question is era and prototye, if any. These answers help determine minimum curve radius, structures, details, and scenery. If you don't state any preferences, then you often end up with a less than satisfying hodge-podge that doesn't seem to have a purpose.
Another question is the type of operations you prefer. Will there normally be more than one operator? How many? If there is usually only you, about the most you can do is be switching with one train, while another "circulates" on a continuous run. Are you the "railfan" type who prefers to watch trains run through realistic scenery? Are you the "dispatcher" type who enjoys making a complex schedule of multiple trains weave their way across the layout in both directions. Or do you prefer being the "engineer" who does everything with one train at a time, making up a local way freight in the yard, switching the industries enroute to the next yard, breaking up the train, putting the engine to bed, and getting ready for the next train?
Settling other more mundane issues will also impact layout design. Will you be using steam engines for which you want a turning facility? Will you be using walk-around control or a central panel? How will you uncouple cars, or will you just stage complete trains? Will you use DCC? If not, how many cabs will you want/need in DC?
Last, I suggest you have a final track plan in fairly good detail before you start on the 1st phase. Although there will be changes as time goes on, at least you will have checked to make sure everything will fit once you get beyond the initial phase. The final track plan will also be a great guide to setting up your wiring plan with the future state in mind. Avoids a lot of rework this way!
yours in planning