Inexperienced cyclist needing help on gearing/crankset/cassette, etc

I would think you would be fine for the race.  I believe the 28 mile loop has a total elevation gain of about 50 feet!  So your gearing should be great.

Now for hills, maybe not so good.  I have a compact 34-50 crank with a 11-28 cassette which seems to give me a plenty low enough ratio with the 34/28 of 1.2 vs your lowest gear which is 39/23 which is a ratio of 1.7.  Your easiest solution would be to pick up a cassette with a lower granny gear.  If you got a 11-28 like the one I have your low gear ratio would be 39/28 or 1.4.  Your 39 tooth ring could be dropped to a 38 but no lower without buying a compact crank set and that wouldn't pick you up much in gear ratio, 1.35 vs the 1.4 with the 39.

The gearing you have is exactly what I have.  

It's designed for strong cyclists and/or generally flat courses.

Here is the annoying thing.  You could get a new cassette (the rear gears) something like 12-28.  This would give you slightly easier gearing.  You would only need a longer chain, so the cost would be less than $100.

The alternative is to get a compact crank.  This would still be 2 rings but the smaller one would be 34T.  That, in conjunction with possibly a wider cassette (as above) would give you even wider/easier gearing.  But a new compact crank is going to easily cost $200-250.  You can't just put a smaller ring on your existing crank.

There are lots of online gear calculators where you can put in your current setups and tweak them to get an idea of what this will get you.

I think the most important question involves (a) what type of riding you will do and (b) your riding style and strength.  

I hope that helps.

Your Cannondale probably had 30/42/52 rings so you had some really low gears available for that climb.  The Kestral lowest gear on the Kestral with the 39 tooth small ring is about 25% harder than you're lowest gear on the old bike. 

You could switch to a compact crank (34/50 teeth).  You'll probably have to take out a few links in the chain, and you * might* look at moving the front derailleur for the smaller diameter chainrings.  This change would make the Kestral only about 10% harder than the old bike (but of course, it's lighter, and fancier, so you get some of that back!) 

Switching the rear cassette is also an option, but you need to pay attention to the total number of teeth you want to shift over.  You already have 14 (39 to 53 on the front) or 16 (34 to 50) teeth.  With a 12-23 cassette, you have another 11 on the rear, so 25 total today, 27 with the compact.  If you put on a 12-28, your poor rear derailleur has to take out the slack for 30 teeth today (14+16) and that might be too much.  (Ask your friendly bicycle mechanic about this.) 

There's not much more to add. Either get a compact crank or new cassette.

My girlfriend is pondering SRAM's new cassette 11-32 http://www.sram.com/sram/road/products/sram-pg-1050-cassette She has SRAM components though.

Not sure if you can mix them without a major headache. You could ask the people at the shop you got your new bike from.

Thank you all for your feedback. Nice to know I've got some options. I'll poke around the interwebs today and see $$.

Appreciate it!

It's not the brand of the cassette in this case, but rather the size of them. Shimano has had a limit of 27 or 28 teeth for many of their short cage derailleurs. SRAM has been able to go a bit bigger in handling a 32. Do check for the specific model you have (or gf in this case) before going bigger though. Shimano can still get there, but a new derailleur may be necessary.

Outside of this derailleur limitation, Shimano and SRAM cassettes can be used interchangeably with each brand.