To me, music for running/cardio needs to have a steady beat. B/c your focus on it will inevitably vary greatly (as you contemplate the degree of pain your body is in etc.), so you have to have something that you can stay with without necessarily paying attention to it. But at the same time it can't be too repetitive or I tend to tune it out a bit and it doesn't help me out at all. So that's my criteria, yours may be different.
That said, my two favourite albums for this of all time are:
And You Will Know Us By The Trail Of Dead: Source Tags & Codes
Stereolab: Switched On (Vol. I, greenish cover)
The first is very intense at times, so it's great if you want to push yourself. But from what you said the second may suit you better. It's great, has a steady beat at roughly the same intensity the whole way through.
I've also been really enjoying Pretty Girls Make Graves 'The New Romance' a lot lately for this. And for techno that's not too repetitive, Underworld's singles collection can't be beat (especially aside from the first two tracks, which were the obligatory 'rarities').
I'll also add that recently getting an iPod (the tiny one) for exercise has opened up a whole new world to me. No more carting around CDs to the gym or holding a Discman while running. It allows me to just put albums, and individual tracks, on the 512 MB that are great for exercise. I find the right choice of tunes can really help in a lot ways.... Probably not news to anyone but me, who just stepped into the 90's. But I wasn't about to spend $400 just for exercise tunes, although knowing what I know now I may have....
Happy running.
