If I am on a long walk or a hill walk I use the Velbon ULTRA-LUXi-SF.
I don't know what size your rucksack is but this fits inside my Lowepro Rover Plus.
It extends to a reasonable size (122 cm ) and folds down to 34cm.
Her'e a link.
http://www.velbon-tripod.com/ultra_maxi.htm
One other thing, I would be a bit wary of using a 300mm on any compact tripod unless the lens had a tripod collar.
Regards,
Cameron