What to do if i discovered out a mass in my leg i have had for twenty years is likely a sarcoma?

No: tumors of the leg are by definition arising from muscle or connective tissue. One can have a large fibroma, or lipoma. By definition, if there is a sarcoma and you leave it alone, it will grow on you. Malignant sarcomas become aggressive over a short period of time and tend to metastasize to lung. If a benign lesion changes growth pattern then biopsy is always indicated due to transformation.