I think 15 minutes is extremely aggressive, especially from the BWI. It's a about a mile from where the walkway begins by the BWV. The average person walks a mile in about 20 minutes. That's not assuming they are puching baby strollers or walking with youngsters.
Considering that there is another 3-5 minutes to get from an Inn room to the start of the walkway, I would think it's more along the lines of a 20 minute to half an hour walk for the average family.
Anne