sounds like a good ghetto set-up at first and i appreciate the reply but i don't see a way for me to constantly increase the intensity with a single-resistance bungy cord and a medicine ball. it feels as though i'll get to a point where i won't be able to increase any more variables- such as speed and weight- to increase the intensity

I guess i'll make it clear that I'm into sprints and middle distance (max 800m) so I'll need to do some pretty intense exercises and lift heavy loads. If I could find some variable-resistance bungy cords or variable-weight medicine balls i'd probably try it.