The AGNS is in a very central location in Halifax, close to the waterfront. This is a great place to walk - with lots of shops, museums, parks, restaurants and hotels all along a waterfront boardwalk - so you won't have any trouble finding a place to stay well within your 15-20 minute walking limit. The issue will be finding something reasonably priced!
Most of the downtown hotels cost at least $150 per night, so (depending on your budget) you might consider that a good price. I agree with Maryann that the Westin is a good choice if you can get a better deal - plus they have really comfortable mattresses and are next to a great little restaurant :) Because the waterfront area is more commercial than residential, there aren't many B&B's in that area.
There is a great B&B across the harbour in Dartmouth, though, called the Blockhouse Hill Bed and Breakfast ( http://www.blockhousehillbedandbreakfast.com/). A friend stayed there just a few weeks ago and loved it - and the singles rate of $75 per night might be better for your budget. It is very close to the ferry terminal in Dartmouth, and the AGNS is very close to the terminal in Halifax, so that might be a good way of stretching your money but still staying reasonably close to your destination.
Hope this helps! Céad Míle Fáilte! One hundred thousand welcomes!