I recently went 8500 miles on the (Ultegra) chain that came with my (Ultegra groupset) bike when it was new (two and a half years ago)... the LBS said it still had miles left on it, could have likely gone another 1000 or more.

If I have any secret or advice about how I got some much usage out of it, it is that I keep my drivetrain pretty much immaculate. Clean my chain frequently (maybe every 700 miles or so; more, depending upon how much grime it begins to collect); keep the cassette and derailleur pulleys clean as well; wipe the chain every other ride or so by running four or five revolutions of the crankarms while holding the chain with an absorbent cloth.

Use quality chain lube - I've been using T-9 for years... developed by Boeing, who seem to know quite a lot about metal moving parts.

I think you would be fine with another 105 chain.