My guess is that while you don't have a structured plan, you were probably a good college runner and have some inherent sense of what you need at any given time and probably have followed some sort of hazy, not-quite-spoken-out-loud plan.
But 2:23 is going to be a stretch for you based on the combination of your half time and your volume, so you are going to need to do something different. Obviously, increasing your training volume will need to be a part of that, and that alone may get you into the ballpark with respect to your intermediate goal. If, however, you are going to take a real crack at 2:18, I think you are going to need to come closer to maximizing your potential, and maximizing your potential means both increasing your volume and squeezing the most out of each mile that you elect to run.
Personally, I lay out my entire training cycle before I start in on my training plan - usually about 18 weeks out from race day. Now, that plan changes as my training progresses, but I really like to have everything laid out in advance as a guideline. You don't need to be quite that structured, but you should have a pretty good idea as to how you want to attack the training cycle and what each phase of the cycle should look like.
For me, that means a block of work at 5k pace out before I get to the heart of my training cycle during the last 12 weeks. I don't necessarily need to lay out every workout for every day during that vo2max phase, but I do know that from 18 weeks out until 12 weeks out, some sort of vo2max workout will be the primary emphasis each week. Similarly, I know that the last 12 weeks will be a heavy emphasis on lactate threshold pace, with the only variation during those 12 weeks being races and an occasional long effort at goal marathon effort. Does it matter if the threshold work is 7 x 1 mile or 3 x 2 miles or 3 miles/2 miles/1 mile? Maybe a little (because if you structure those in the right order, you can safely build up to a higher total volume at LT pace than you might think), but at least focusing on what the overarching goal of the then-current phase will get you pretty far along toward your goals.
If you don't have a good sense of how you would like to set up a training cycle, or what the goals of the phases should be, then you should consider hiring a coach or at least buying some of the more well regarded books out there. I have based my training cycles on the principles laid out in Daniels Running Formula (2nd edition), and I have used those principles to design training plans for male athletes who have run 1:04 for the half and 2:18 for the full and a female athlete who ran 2:45 for the full. So I can tell you that those principles are solid and sound; but there are certainly other approaches that will work as well.