I just got back from Belize and since my sister wants to come down next March and is REI I checked out Banyan Bay for her. I also took some good pictures of the place that I will have on my web site soon. It is a beautiful property.
To answer your questions: NO, you can not walk to town from there after having partied.
However, any of the places that have the wide beautiful beaches are not in walking distance to town. So you have to make a choice.
But, the good news is - Taxi's are cheap - $5.00 BZ for any trip. That is $2.50 US
I stayed at Hide Away, which is about half way to Banyan Bay. If I walked early (before 8:00 AM) and went via the beach, not road, it took me 15 minutes to Ruby's. Therefore I would say BB is about a half-hour walk. I would not walk it at night or in the heat of the day.
There is a reason it is called "high season." In low season everything is less expensive but you take your chances on the weather.