For some context, the document http://www.history.army.mil/html/books/075/75-1/CMH_Pub_75-1.pdf http://www.history.army.mil/html/books/075/75-1/CMH_Pub_75-1...

For some context, the document http://www.history.army.mil/html/books/075/75-1/CMH_Pub_75-1.pdf http://www.history.army.mil/html/books/075/75-1/CMH_Pub_75-1... indicates on page 50 that in December 1860, "on the Eve of War", the army had a total strength of 18k men, of which 1k were commissioned officers, of which four were generals. A different source ("Armies South, Armies North" by Alan Axelrod, also author of the more compellingly-named "Generals South, Generals North") provides more detail, including names: Winfield Scott, William Harney, David Twiggs, and John Wool. This and other sources also include Joseph Johnston, then quartermaster general of the US Army, as a fifth general. There were also two more brevetted generals: Albert Johnston (no relation to Joseph) and Newman Clarke. Maj. Gen. Winfield Scott (the commanding general of the US Army) was 74 years old and indeed unable to ride a horse. However, all seven of the generals had seen combat. In particular, four served in the War of 1812, all served in the Mexican-American War, and all served in the Indian Wars. With the exception of Joseph Johnston, all of them even served in war as generals (at least brevet, but usually full).