LHot - They lie about some of the 8's I swear. I wouldn't blame you for failing one, especially after giving blood. Yikes! o__o
Ok, so the reason your legs get tired is because DDR is both aerobic and anaerobic. Yeah you get out of breath and your heart beats faster because it's cardio, but there's also a lot of anaerobic exercise, particularly when doing 10 footers/lots of jumps. The reason it hurts is because lactic acid is building up in your muscles, and that stuff doesn't go away very quickly.
I agree with slackbot though, just try to push yourself.
Besides, if you really want to be good at DDR you have to practice. You can increase your stamina and improve your PAing skills at the same time, so why not?
Oh and bag is not stupid.
Try doing Oni (Challenge Mode) courses, most of them are on x1.
Bag on Expert isn't that bad because there's no weird jumps. Try it on Standard x1, you won't be able to tell what's a jump and what's not, especially on Solo.