I stuck a marzochi DJ3 fork on a crappy dual suspension bike, and the rear suspension became SO apparent! every time i pushed a pedal it would just compress.
I rode my brothers crappy Raleigh Tora on a 42km backcountry ride loaded with a tent, sleeping bag, and a bunch of other crap, then camped and biked out the next day. Even with deep mud on one damn trail (about 2km of knee deep mud we had to walk.. &%#$!!! ), and a bunch of water crossings, it faired quite well.
Crank the rear suspension spring as tight as it will go, and maybe search ebay for a cheap but decent fork. (dont buy a long travel one though)