Running is high impact, and it can take a while for your leg muscle and tendons to adjust to stresses and impact of running. 12 months is probably a reasonable timeframe to train towards a marathon.
But probably the best way is to set yourself some intermediate goals. Start with running a 5K, then a 10K, then a half marathon before setting out to tackle the marathon.
In terms of getting to 5K, I'd definitely recommend a Couch to 5K training programme. Rather than running continuously, these programs work through progressively increasing intervals of running and walking.
Training plans for all these distances can be found at www.coolrunning.com/engine/2/2_4/index.sht