I'm on a 6 day a week training program. The program calls for the long run on the last day of the week before a rest day.
I've been doing my long run on the second to last day, then a light run (4-6 miles) on the last day before the rest day. Is there any risk to that? Is it essential to take a day off directly after a long run?

No, but just make sure your long runs are pretty slow, as Im sure you are already doing. Ill usually do an easy 5-6 after a long run after a long run and havent had any problems. I am; however, not an expert on the marathon and perhaps Formationflier has more to add.

The most important thing is that youre getting it in, long runs are the cornerstone of any distance program. Some weeks Ill only be able to do a long run on a wednesday or thursday...right in the middle of a 60 or 65 mile week, so that means lots of miles till the next rest day.

Good luck!

no disagreement here. make your adjustments as you need. Any schedule can be tailored as long as you retain the key parts.