I'm a 3-season runner, so I'm rather concerned when it comes to the upper-body portion of the CFA. I can do a decent number of pushups, and am pretty sure I can work my way up to a competitive score simply by doing more pushups, but have had trouble when it comes to pull-ups (due to my weak arms and heavy legs, I find it difficult to do one). Is there any exercise that can increase the number of pull-ups I can do?