How long must long runs be for marathon preparation?

runningwife.com - Hansons-Marathon-Method-Review: "You will be OK capping your long run at 16 miles. The reasoning behind the 16 mile cap during training is to help keep you healthy".

Others prefer my long runs are done by performing several marathons in a year.

sarakurth.com - Why you should consider running multiple marathons a year: "A recurring injury is a common reason some people can’t do multiple marathons a year. But the lessons and ideas apply to training for multiple races of any distance, not just marathons.".

川内優輝選手、スーツで快走🏃🏃#久喜#マラソン大会 #川内優輝 pic.twitter.com/6ZgpHYi7xw

— ぴの (@pinobubu) 13. März 2016