http://www.conitec.net/beta/amickey.htm
It is 'mickey.z'.

'CamDist += mickey.z * time_step * 10;'

is sufficiant for both, increasing and decreasing.

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt