As people above said, the aches are from the residue of lactic acid buildup resulting from exercise to the point where your muscles were stressed – a good sign that you worked out hard enough to tell your body to start growing stronger. The exercise magazines are full of ads for expensive post-workout supplements that will add 50 pounds of muscle in a week, enable you to leap tall buildings and eat nails, etc.; all of them just give you expensive urine imo.
The advice re potassium is spot-on, I think; try to get it into you withoin an hour of the workout. You can get it through a meal seasoned with “salt substitute”, which is potassium chloride, a banana, a protein energy bar, a protein drink, Japanese seaweed crackers, whatever – just look for 100–200% of your RDA on the label, as more won’t hurt you AFAIK. But also try drinking an energy drink – preferably something with carbohydrates like sugar – for energy and hydration beginning about 15 min B4 you start and through the workout.
And – finally! – the answer: go to the gym the very next day afterwards instead of waiting two days. %^> You’ll hurt at first but it will pass during the workout, even if you exercise an entirely different muscle group (which you should be doing). Simple and free. And the post-workout pain is the sign that you’re benefiting; there’s no free lunch.