When barrel aging, it's usually the flavor from the barrel that you're after. You mentioned that you poured some whiskey in for sanitizing purposes. This is fine. I've done the same thing myself. But I would recommend giving the barrel a good rinse before putting your beer in. I didn't do this once with a barrel-aged IPA, and it came out tasting like someone just poured some whiskey in the beer. Unless of course that's what you're going for.
Usually I heat up enough water to fill around 1/3 of the barrel. I heat it up to almost boiling temperature (sometimes I add a sulfite too). Then dump it in the barrel, shake it around for a few minutes, then drain it. This will do several things. Makes sure the barrel is rinsed of debris. If you're using really hot water, it will help sanitize the barrel. It will also show you if your barrel has any spots that are leaking. In which case you should let the wood soak and expand for a little bit. I'll then let it cool down before putting anything in it.
From your description it sounds like you're going to put 10 gallons of beer in a 15 gallon barrel. Just forewarning that this will leave your beer exposed to any oxygen that's in the barrel with it. Usually you want to treat a barrel the same as a carboy, etc. You want to leave as little air space as possible. It will mean brewing another batch, but I'd recommend getting that thing filled up.
Periodically taste a sample and pull it as soon as the flavor is how you want it.
I agree with kramerog, the maple syrup probably took most of the bourbon flavor. You're probably going to get more of a wood aged flavor. Still great stuff!