Too fast pace - you should be able to carry on a conversation while you run.
Too fast ramp - 10% is a general rule although if you are starting very low you can do more and when you get higher 10% may be too much (if I tried to do 10%/week for more than a couple weeks I would die).

The important thing to remember is that your cardiovascular system adapts MUCH faster than your joints and ligaments and bones. So you FEEL like you could run farther and faster but you get injured. Since you do a lot of elliptical you probably have good cardiovascular but you are not used to the pounding of running.

Always finish feeling like you could do more.