There is indeed a running race at Yankee: http://yankeespringstrailrun.com/wp/winter-race-information/
They are using quite a bit of the mountain bike trail for the event. There is a link to the trail map on the info site. The cut-off for the race is 7:00 p.m. I guess you could go.....and it wouldn't be quite like running into a horse at Fort Custer head-on during a Time Trial...but, Sunday may be a better day.
As far as conditions: We have more snow down here than in GR. Today will kill it off pretty much. I would expect it to be somewhat sloppy today, but by Sunday it should be sweet. I rode out there last Saturday and the conditions were fantastic.