I also started running with my race backpack, donated generously by Bindweld Plastics http://www.bindweld.com/!
I am carrying a few can's of beans and my sleepingbag plus 2 l of water in a camelbag. Guess it is around 5kg which is half of my planned race weight for Morocco. Very different running with this bag on the back and I am still finding out about all the spots I need to protect well with tapes for the race - 7 days of chafing mixed with sand won't go done well in Morocco.
Saturday off to a fun race from Otford to Bundeena - 29 km sounds like a stroll in the park but I have been warnend to expect some steep and hard trail running :-) http://www.coolrunning.com.au/fatass/hq/index.php?title=Otford_to_Bundeena