Jamie there are several good areas to fish the San Marcus River. The Lulling area has some good bank area and has also got some shallow wading spots. You can Google to pin point the areas. Also there are some bank areas between San Marcus and Lulling that hold some nice Rios.
Side Note: There is also a City Park just off of IH 35 where the San Marcus River runs through it. I have caught some nice Redbreast and Rios there. Good luck.