Tundra_Man 

For the average person who does no physical activity at all, any amount of cycling is beneficial. Just start riding. Even around the block is better than not going around the block. You can always go farther as you get used to it.

Any person moving from the "couch potato to somewhat active" state will go through a period of sore muscles and joints as their body adjusts. Once a person gets through this point and are active I think very, very few people actually overtrain. Overtraining is certainly not something you're just going to accidentally do without realizing it. I've had some injuries resulting from overtraining and the sheer volume required to get to that point left me with little doubt in my mind what I was risking. I made a conscious decision to skirt the edge of what my body would handle and I went over it a few times.

I'll hear people say, "too much exercise isn't good for you either" and "I don't want start working out and wind up looking like a body builder," but they either have no clue what it takes to get to that point, or are just using it as an excuse to remain sedentary.