If you are planning to run a marathon or half-marathon this year, then one of the first questions you will ask yourself is: do I have enough time to train for it? Two things that can help you answer this question are here: blm849/Bernie-s-Race-Scheduling-Spreadsheets: My Race Schedule Spreadsheets to plan out my training runs.
With my spreadsheets, you enter a date, and it will give you a 16-20 week schedule you need to follow to get ready for a marathon or a half-marathon (or a 21K, as I like to call it).
Since they are spreadsheets, you can adjust them in any way you see fit. Add weeks, change the mileage, etc. If you have any other changes you would like to see, let me know.