Marathon Training plan question

All of the marathon training plans I have looked at put the long run (20 miles for me since I would be a 1st timer) three weeks before the race.  Is it a huge deal if I did the long run 4 weeks before the race?  I'm looking at a calendar and the 20 mile run is the same week I will be away and probably not able to do a 20-miler.  If I bump the schedule back by a week everything fits much better on other weeks as well.

Thank you!