I use my Pentax 645N the way many use 35mm. It's just a little heavier, but I can still carry it out in the woods a decent distance. I tend to use 35mm if I don't know what I'll find or if I'm shooting mainly snapshots. The 645 goes if I have a reasonable suspicion of finding something I'll want to enlarge. If I know I'll find something on an easy trail or close to a parking lot, I'll take the 4x5 (it's a monorail). I just picked up a speed graphic, so I may take that a little further into the woods.
Since I also have a currently 18 pound baby to haul around, that gets balanced against the camera weight. I'm looking for off-road strollers on craigslist.