My lazy method for the above would have been to dial in +1.0 ev, knowing that the subject was backlit and darker than the majority of the scene.
However,
The built in flash would cover that distance quite well.
Using built in flash would work better than exposure compensation, because it wouldn't blow out the background.
You would have to have the aperture quite open, say f/4 to f/5.6, as it's aperture that affects the range of of the flash, not shutter speed or ISO. Trick is not to over expose, because the shutter speed will be limited to 1/180 - so just watch out for that.
If you took the above shot in RAW format, it's easily recoverable, especially with something like a K5... Just move the exposure compensation up by about +1 in PP.
Edit:
PS - there's some dust on your sensor... see the light grey circles? Follow the instructions in your manual to clean the sensor, with a
puffer or even better the
Pentax Image Sensor Cleaning Kit O-ICK1.