I live and die by Jeff Galloway's Run/Walk/Run training method. The purpose of his program is to get you to finish (and not necessarily break any running world records) your marathon by not injuring or killing yourself to get to the finish line. Here's what I did to get myself ready for the Disney Marathon in 2017 (although I did not finish it due to injury and other things that was going on at the time, too.)
1) Start out by participating in your local 5K races - just get out there - don't worry about speed. Just get out and have fun.
2) While you're playing around with your 5K races - start looking up Galloway's Run/Walk/Run method - he'll start you off with 30 second runs/30 second walks. Then, eventually you'll work your way up to 4 minute runs / 30 second walks, but do it easily and when your body is ready for that.
3) Once the run/walk/runs are getting longer and you feel more confident in your running abilities, then take a swing for some 10Ks (6.2 miles). Get out there and have fun - don't kill yourself getting to the finish line.
4) Continue using the run/walk training method - start pushing for longer distances (10 miles)
5) Then, if you get really brave - do a half marathon or two. The first one will hurt. My first half marathon, I ran it in the heat and humidity in Florida and was miserable for a couple of days afterwards. But, it was all good! I rehydrated, rested, recovered, and was ready to run again!
6) Continue with the run/walk training method - start getting into that 15-20 mile range. Then, once you hit the 20 mile range...you can start to back off on your training for the marathon. The marathon (26.2 miles) is doable. Just pace yourself. Hydrate often. You'll be fine!
This should help you, too. Look at the training programs for the different RunDisney events and follow them. If you have never ran before - I strongly suggest starting now (pre-training) just to get your body used to the idea of running. Then, you can start your actual training according to the training schedules.
https://www.rundisney.com/training/running/
Happy Running!! See you all on the course in 2018!