Vanessa will be leading this Saturday's run, as I will be in Big Sur for my marathon. She has also extended this meetup to the Boulder Women's Running Group, so you'll get a chance to meet people from both groups. There's a bit of elevation gain, but you can always hike/walk those sections.
Vanessa, Andy and Patti are all planning on running at least 12 miles (some up to 16), but rest assured you can set your own mileage and turn back at any time, or do the loop with the group. You'll have plenty of options.
The general plan is to run the South Boulder Creek Trail, then connect to Mesa. Vanessa will have a route planned out.
I highly recommend this Google Pedometer site as a user friendly way for you to chart your distance on any road or trail: http://www.gmap-pedometer.com/