As many mentioned there are rules of no more than 10% increase a week. I felt (yes past tense) the same as some on here that you should ignore that - man up and just ride. So I bought a new road bike (upgraded off of a hybrid) and so fell in love that I went from riding 100 miles a week to 200 miles a week. I also broke another rule about modifying your own fit. Due to breaking these two rules I fell victim to "Handlebar Palsy" (where you have nerve damage in your hand that can be anything from numbness to losing use/control of fingers). This started with just numbness and I basically ignored it and kept pushing - in fact pushing harder. This lead to me loosing use of my pinkie which then started concerns and a trip to a sports doctor. The answer was to "stay off the bike for at least 2 weeks".
So I hope that someone can learn from my stupid mistakes and not just ignore these rules of thumbs. They are there becuase others have preceeded us and learned lessons that can help as we try to follow in their footsteps.