Check out this plan if your priorities are weighted more towards getting across the finish line than your finishing time.  This is great for first or second time marathoners or novice runners.  This plan includes an explicit basic training phase for those who cannot yet run 3 miles continuously.  Yes - you can go from not being able to run 3 miles to running a marathon in this 21 week plan! So you ask how much will I need to run during training?  Expect at least 4 days a week of running, 2 cross training days, and 1 rest day.  You can definitely modify your cross-training days to rest your body as needed.   Your peak week will reach 33.5 miles total for that week and before the end of the program you will complete a 19 mile run (several weeks before your race)!