You reach Forest Road No. 578 via Forest Road No. 780, Forest Trail No. 488, and Milk Creek Trail No. 474.
After a mile or so, a climb of staggering difficulty begins. At least you know you're gaining elevation rapidly, as you hike-a-bike over a rockpile masquerading as a road. There are one or two places that are rideable, which gives one hope, but just as one has gotten into a rhythm, the ultra-steep, ultra-rocky aspect of this road reemerges. It doesn't let up until you reach Sawmill Park and encounter a much wider, smooth, and relatively level dirt road.
Ride this easy road south until you reach CT - Segment 16: Marshall Pass to Sargent's Mesa.
Turn east and head gently downhill until you reach the junction with Tank Seven Creek Trail No. 480. You can then descend that trail or continue on the CT until you reach Marshall Pass.
I would never repeat this ride except at gunpoint, but am listing it (a) so people know what to expect and (b) in case people want to descend it. For riders with advanced technical skills, I'd guess most of it can be ridden downhill until you reach Milk Creek Trail No. 474. At that point, go west, find Trail No. 488 and Road 780, ending up in Sargents. Good luck!
1. Some of this route shows as Forest Trail No. 625 on the generic 2014 Forest Service map, and all of it shows as Forest Road No. 578.2 on some editions of the National Geographic Trails Illustrated map No. 139 — La Garita–Cochetopa Hills.
2. The National Geographic Trails Illustrated map suggests you can also ride or drive up Marshall Pass to an intersection with this road along Milk Creek. However, as the map also shows, you'd then be riding through some off-limits private land on your way to the junction with Milk Creek Trail No. 474. So, avoid this approach unless you have permission from the landowner, a nearby ranch.