There is no such thing as over training! As humans, we are designed to push our bodies to the very limits.
Back in our hunter gatherer days, we needed to keep running and running, just so that we could eat and not be eaten!!
Our bodies have evolved through time to become the most dominant species on the planet, but it cannot sustain this dominance without a bit of help. The problem we see is that people tend to under-recover. They do not give back to the body what they have taken out of it. Think of your body as a bank - every training session withdraws more and more money but you will eventually go broke if you don't deposit anything back! This comes in the form of treatment, massage, mobility, stretching, yoga, spa, sauna, sleep.
A good skill to develop is being able to listen to your body. You could be pushing your body day in, day out but if you ignore those little whispers of niggles and pains that your body is presenting you with, something will eventually break down. The whispers overtime can easily turn into screams and cause some serious damage and injury. The body is sending these whispers out for a reason. The brain is requesting that you change something before its too late!
The more experience you gain through movement, the more you can start to understand the differences between muscle soreness and muscle pain.
The easiest way to help differentiate between the two is that muscle soreness will start to disappear with movement, where as muscle pain will continue to stick around. Do yourself and your body a favour and listen to its warning signs. It could mean the difference between and little niggle injury and a long term issue.
Most top athletes today have some kind of recover routine and if they want to continue to perform at their peak, they don't ever skip it. A lot of top athletes are similar in their skill sets and talents and so when they are looking for that extra edge over their opponents, recovery is number one, as the athlete who can recover and set them self up in the best possible condition for their next session, game, event will have the biggest advantage!
A good way to look at it is when you finish a hard gym session or a grueling run, don't think about cooling down from it, but instead, preparing your body for the next session. So keep your books balanced and start giving back to your body.