Well, FWIW I didn't follow a rigid training plan for my 26.2 or 50k this year.

In training for the marathon I printed off a program from Runner's world and modified it to what my body told me. My primary goal was to stay close to the weekly mileage it suggested & to make sure I had an easy, fast & long run each week. Speed-work isn't my idea of fun so I didn't do any. The only real speed/tempo runs I had was trying to keep up with the faster runners in my Thursday morning group run or when my runner's high said "hey, lets see how fast we can sprint & for how long". I never ran more than 3-4x a week and never more than 25-30miles total per week. My longest run before the marathon was 18 miles.

Did it work? Yep! Absolutely. I finished only 5 minutes behind of my A goal. There is no question in my mind I would have been right on target had I stuck to my hydration/fueling plan, but that mistake was beautifully corrected during my 50k & 27miler (A few wks ago.)

Would/Will I do things differently next season? No. I believe that training for a race should be just as enjoyable & at times just as hard as race day. That way if my race doesn't turn out to be everything I hoped it would be I'm not supremely disappointed because I had so much fun in the runs leading up to it.

In short, yes you can complete a marathon without a rigid plan. There is a difference between being half-assed in training or being more free-spirited about it though.

Just make sure you aren't lazy about the training. Still push yourself to see what you are capable of. Be prepared to run with your mind when your body starts to make you doubt & vice versa.

There is one thing I have to say about you awesome racing ladies on TE is that once you set your mind to something, you conquer it! Good luck!