When I read your post I interpreted it as saying that the kenyan hills, farlek training etc puts you off because it baffles you and you aren't sure what you should be doing - not that you find the sesisons "too much to bear" or "can't be bothered".
I totally identify with this. I used to stress ahead of a session about what I was meant to be doing, during the session about whether I was doing the right thing in the right way, and after a session worrying if I had done the right thing. It took all of my energy worrying about the plan rather than stressing about the really important things (do I need gels, what should I have for breakfast, how can I get some more sleep
). My most successful marathon was where I took a relatively simple training plan, and re-wrote it to only include the miles required (taking out all talk of threshold, 8 by x minutes intervals, kenyan hills). I then made sure I did the long runs at a sensible pace and made sure that I covered the mileage.
So to answer your question, yes I do think you can 'just run', but I do think that you need some structure (as Run Wales says, a marathon is a different beast to a half marathon) in terms of miles needed and breaking these across a week.
People will say that you must include speed, hills etc etc and they are right that is the only way you will run what you are capable of. But I also understand the idea of been happy to finish a marathon with the target of finishing. I think that is different from saying that you are content to run-walk in 5:15 when you might be capable of sub-4. There is a middle ground.
I would say that there is a lot to be said for taking a cautious, sensible approach to a first marathon. There are enough things to worry about that just going through it for the experience is valuable. You then have your benchmark, and can then look to step up a plan to incorporate some speed work etc the next time round when you have more confidence.
So in summary, I would say doing the miles is the priority, and for that you need some structure. But can you get round a marathon without having done the "value added" speedwork etc - yes, I think you can.
Of course, you could also join a running club where you will do structured sessions with others, and will learn about how it all comes together.
As to whether the MK marathon is too soon - I think it's probably ok, but you need to sort a (milesage) plan soon and get on track.
Good luck!
ps if it is just that you can't be bothered to put the effort in, then yes, you are doomed
.