From the F street bridge, head over across the tracks to the old paved road and head left/upstream/generally west. It is about 1.0 mile from the gap in the fence on F street until this road meets the better pavement of Co Rd. 175 at an intersection.
Keep heading straight/upstream (crossing rr tracks) for .234 miles on CR175 till you come to another intersection with a stop sign. Turn left here and go downhill, crossing the bridge over the river, until you hit the pedestrian crosswalk for part of the Salida Trail system in .1 miles.
You have gone 1.334 miles from the North end of F street to this point.
Here, you can head left less than 100 feet through the gap in the guardrail until you hit the gravel trail around Sand lake. It is .445 miles around Sand lake on the trail (watch for ducks, geese, 2 little drainage ditches and fisherpersons).
At the pedestrian crossing, you could go right and head upstream on a nice gravel trail along the river.
From the pedestrian crossing on CR175 it is .489 miles until the trail turns right at the top of a hill by some big rocks to go parallel to CR 154.
The trail continues upstream along the edge of or just off CR 154. It is another..503 miles until you go up a little hill and around a corner to reach the SE corner of Franz Lake. (by the bench).
So it is almost exactly a mile between Franz and Sand Lakes via the trail.
Now you have numerous options.
One rectangular trail loop of the overflow pond you just passed east of Franz lake is .278 miles.
A loop of Franz lake itself is .60 miles.
Or continue straight .161 miles past the bench along the south shore of Franz lake till you hit another intersection. Going west here is a new trail section with very smooth footing which will loop back around an open field to join the Franz lake trail. This area was formerly used by the gun club. In the shape of a big “U” there is .451 miles (or `~795 yards) of new trail. Or about .60 miles (~1050 yards) using the new ‘gun’ trail and the west trail of Franz lake to form a loop.
There are various sections ( marked off with lines of rocks on the inner edge) of the gun trail measured in segments of 220 yards for those wishing to run faster intervals/repeats.