Your ignition will keep firing only as long as the battery has good voltage. Without the battery charging, you're going to be limited.

 

As above, the car will still run fine for a while but will eventually fail. There is a very good DIY guide on here for replacing it that I posted in when mine went.

 

My understanding is you are running on the juice left in the battery. Once thats gone the car will stop. Mine did 30 plus miles but not sure how new my battery is. The workshop confirmed the 996TT has so many electrical devices its not surprising you wont get more miles. Not sure how S4gotten managed a few days driving, but not saying he didn't. Still think you will get your 23 miles OK.

 

I just drove it 7 miles in stop-and-go traffic and the voltmeter needle was at 12.5V in the beginning of the trip after overnight charge, dropped to a hair below 12V at the end.

The car made it safely to the shop and hopefully I'll get it back tomorrow.