Low blood pressure and low heart rate in runners. Am I at risk for an abnormal arrhythmia setting in?

No. Very well conditioned athletes typically have low resting heart rates and low normal blood pressure that are a result of their excellent physical shape and not because of underlying heart disease. Now, if you are feeling faint, dizzy, light headed, weak or unwell in any other way seek medical evaluation.
Generally no. Generally, these are signs of (especially good) health in runners. If you're otherwise feeling good, i wouldn't give it another thought.