A marathon is 42.195 km, not a longer version of your usual evening walk. The first decision is whether you have enough time to prepare, not which shoes to buy. If you are new to running, start with a walk-run programme and build a comfortable running habit before beginning a marathon-specific schedule.
Choose a race around your current fitness
Write down the runs you actually completed during the last month. If that list is empty, make a short-distance event your first milestone. An established beginner programme such as Hal Higdon's Novice 1 lasts 18 weeks and already starts with a long run of about 9.7 km. Those 18 weeks are not a couch-to-marathon promise. Allow extra months to build the base, and consider the event's cut-off time.
Give the week a repeatable shape
A marathon programme generally combines easy runs, a longer run, recovery and sometimes cross-training. Put the long run on the day you can also eat, rest and sleep properly afterwards. Keep most running conversational: you should be able to speak in sentences. Follow one suitable programme instead of combining the hardest sessions from several.
Progress without chasing a magic percentage
Increase training gradually, but do not treat any weekly percentage rule as a guarantee against injury. Fatigue, previous injuries, terrain and heat all matter. Repeat or reduce a week when you are struggling. Do not cram missed runs into the next weekend, and do not run the full race distance just to prove you can.
Make finishing healthy the first goal
Practise footwear, breakfast, fluids and fuel on long runs. Protect the lighter final weeks of your chosen programme. Seek medical advice before starting if you have health concerns; stop for chest pain, faintness or severe breathlessness. FitGeek can support strength and meal tracking, but a general fitness plan is not a specialist marathon prescription.