Kestrel, you say you struggled mentally - can you expand. Where did the training fall apart last time.
I would build a decent aerobic base between now and starting a 16 week plan. Lots of easy miles. Start building your long run distance now. Do some easy midweek runs, incorporating a few 100m strides on some of them to get you legs turning over faster.
I develop my speed through lots of miles, and one tempo run per week. Once you have a base, add in the Tempo run. Mine is something like 2 miles warm up, 5 to 7 miles at tempo pace, 2 mile warm down. Tempo miles are a little quicker than my half marthon pace. To get started just do 2 or 3 miles. These are hard workouts, allow recovery time to reap the benefits. Long intervals is another option - doesn't suit me.
Run some of your long run miles at your target marathon pace occassionally.
Don't run all of your sessions at the same pace.
With the right balance of sessions, putting in the miles and hardwork, I am sure you will improve. Swimming and biking can be useful on recovery days.