Build up a base first... then.. if you just want to finish a marathon, be careful. A lot of the plans out there take into an account a base mileage which may be higher than what you are accustomed to... and this can lead to injury. The minutes theory really works well for those that just want to finish a marathon. But the demands of taking on a marathon don't always result in weight loss... the increased miles does sometime lead to increased hunger on rest days...so be careful... I know a lot of marathoners that GAIN weight during training because of this. Every mile walking burns approx 80 cal/mile for females... and running a 10 min approx per mile pace burns approx 115 calories per mile. The plans that I see that really work for first time NONRUNNERS use the minutes theory... I'd suggest one of those! Good luck!