Going the distance on my worn out mountain bike.?

The terrain is unbelievably hilly around here, even in towns. I routinely ride around thirty miles on my Mongoose Vadium 26 inch, 21 speed mountain bike. I've already replaced the derailleur, weakest link by far, with and old metal one of of a cousin's grandfather's fifteen speed, it's a bit jumpy, and not all of the gears work, but it's pretty solid. I've also tightened the spokes down, though the rim is warped, so I had to pull the back brakes off. Anyway, despite the problems I've had, I've got no real complains, the seat's set high and been changed to one I can comfortably ride for nine hours. Plus, the shocks soak up even the worst potholes and speed bumps without my teeth rattling. My question is, are there any other changes I need to make on this bike for long distance riding? Perhaps even taking it cross country to visit some friends? Most of what I've found is merely people discussing which BIKE to get, but I'm quite happy with what I have, and also too poor to cough up a few hundred bucks at once.How much would more durable rims and a new derailleur run? The derailleur I'm sure would increase my distance since I'd have nine to ten more functioning gears, and quite frankly, the roads around here are even rough on the standard adjustable spoke mountain bike rims. Would something other than the standard twist gear shifts make a difference? I know that I tend to shift hard and sometimes throw the chain off the sprocket and my hands will get blisters if I'm out more than five or six hours. Anyway, any advice would be greatly appreciated, this is my main form of transportation after all!
I'm appreciating all the answers, but the thing is. I'm broke. It's taken me a month to save up twenty bucks doing odd jobs. Hopefully that will pick me up a new inner tube, and if I'm lucky, a head light to boot. A new bike is out of the question until I can find a job.