Battery efficiency questions in the context of running

the battery gauge was red and very thin

Well, at least in testing here with the GPS on (in GPS only mode) and with a lock, it draws 20mA, so the 175mAh battery should be able to do at least 7 hours.

However, I guess I'm not factoring in the battery usage from the Run app updating every second, so that could definitely be a bit of a hit to battery life.

Also, the increased power draw will actually drop the battery voltage which will make the battery appear more empty than it is. It'd be interesting to see what the battery level ends up showing 1 minute after you've turned off the GPS and run app - it might be a little higher (although maybe that only accounts for around 20% capacity).