As a long-time teacher, as well as a distance coach, my first question would be "What are you trying to accomplish with the extra credit offer?" regardless of the standard you set.
If, like tests many of us encountered in PE classes of yore, college included, your physical fitness test is given as an end in itself, unrelated to the activities that routinely occur in class, any rigorous standard simply rewards the talented for being talented, just as being elected Prom Queen simply rewards the pretty and popular girl for her good genetic fortune. The trouble with such reward systems are that they reward that which is intrinsically rewarding, i.e. being genetically gifted in one way or another, without necessarily motivating desirable goal-oriented habits on anyone's part. The average kid, watching the pretty girl being singled out for her looks and the swift kid getting easy medals, learns little except a cynical acceptance that the rewards in life are passed out to the genetically lucky, and since that is not him, effort is pointless. That is a lesson we should be careful our rewards systems do not reinforce.
I had two college weight training classes where the final grade depended on a mile time trial. The term I lapped the entire class was the one in which I tore a shoulder ligament, and consequently spent the bulk of every class session on the track. The question one might ask is, if I deserved my "A" by running a sub 6-minute mile, why was I the only person in the class spending class time preparing myself to do so, and if the point of the class was to reward diligent pursuit of whole-body muscularity, why were the reward systems designed such that I could earn the highest grade without doing that? The moral to that story is that our reward systems should be congruent with the behavioral habits we wish to reinforce.
I would argue that whatever you measure that has an impact on final grade should have a SERIES of graduated standards that offer something achievable for everyone who trains diligently to achieve them. Then, of course, your class activities should offer structured opportunities to intelligently train to meet those standards.
As a coach, I'm sure you realize that a girl who begins the season running 15:09 for 3000m and improves to 13:15 has achieved something to be proud of, but that same standard would not motivate a girl who had run 11:28.
The trick is to keep challenging but achievable goals in front of EVERYONE, along with a reward system that keeps everyone motivated to strive for them.