I've followed a few different plans, but I'd disagree with the poster who says don't 'plod out 18/20 miles aimlessly'. There is a lot written about the benefits of 'slow running', and particularly running your long runs at much slower than your intended marathon pace. Just to give you 2 examples - one person I know did all their long runs at 11 min/mile but ran 8 min/mile for the race, the other ran long training runs at 10 min/mile and took a whopping 15 mins off her PB, running 7.10 m/mile on the day. Clearly they both did other runs in the week at marathon pace and faster.
The problem with doing your long runs at marathon pace, especially at the business end of things when you're knocking out 18/20 miles is that your body simply can't recover sufficiently to train effectively for the next week. The point of the long run is all about 'time on feet' and getting your body used to that side of things.
I started doing 'slow running' a few months ago - taking all my long runs down in pace, to about 2 mins/mile slower than I'm used to. I managed to run my fastest half marathon for over 2 years in Oct, having not run any long distances at that pace.
Whatever you embark on, do a bit of reading around it first. It can be very easy to fall into the trap of just knocking out a load of runs without thinking about what you're gaining from that run.
Asics have quite a good site where you can put in your current stats (re pace, age etc) and also what you want to achieve in terms of time, and how many times a week you can run. It then generates a plan specific to you. I did this once, but didn't follow it properly - it had runs at 'slow' and 'comfortable' pace in there, but I stupidly ran everything at the same pace and then when push came to shove on the day I wasn't prepared - I'd done more miles than the plan and most of them at a faster speed than I should have done, and the last 6 miles on the day were utter hell.
For my next marathon, I'm using an Athletics Weekly plan. I like the look of these. I fully intend to flex it though - like you, I can't commit to a long run on a Sunday every week, but I'll fit it in somehow. I'm also doing some bits and pieces from the plan now, even though it doesn't start until the end of Dec.
The key thing is to find something that fits around your lifestyle. It has to be realistic. That's what was good about the Asics plan. If you can only run 3x per week, you need to find one that has 3 runs, and those 3 runs have to count.
If you can find someone to train with, that can help too. Or join a local running group. Helps with motivation enormously!