Here is what wikipedia has to say about the plantaris muscle:
It is composed of a thin muscle belly and a long thin tendon. It is approximately 2-4 inches long, and is absent in 7 - 10% of the human population. It is one of the plantar flexors in the superior compartment of the leg along with the gastrocnemius, and soleus. The plantaris is considered an unimportant muscle, it mainly acts with gastrocnemius.
Here is some additional info from a site called wisegeek on "what is the plantaris tendon":
Plantaris tendon ruptures occur more often in male athletes, particularly part-time athletes, between 40 and 60 years old.
When the injury occurs, the individual might actually hear or feel the tendon pop and feel an intense, stabbing pain in the calf. Pain accompanying a plantaris tendon rupture is persistent and lies deeper within the calf muscle than if the muscle itself were strained. In some cases, the plantaris tendon and the plantaris muscle have suffered chronic swelling that finally manifests in a rupture, but most of the time, the injury occurs suddenly.
And I was told that I can walk or run based on comfort. When I mentioned TCM in two weeks, my Dr looked at me and said it is a comfort thing. Granted his look was one of subtle perplexity and although I am not planning on running it as of today, if I can start walking normally in a few days, well who knows. At least I have some hope of being ready for the 50k at Surf the Murph. That said, I did just read another article on plantaris muscle injury treatment that wasn't quite as optimistic, it said one should take off 4 weeks before resuming running so I guess I will ponder things as I start to improve.
So as of today, I will simply continue to do the RICE thing and take it day for day until I am back once again on the trails, hopefully sooner versus later.