In the 1861 census, Elizabeth Johnston is age 20 and her daughter Mary is age 9 months. So if John Johnstone had died by then, it had to be no more than 18 months previously, giving a death date of 1859.
Mary Johnston's birth was registered Jan-Feb-Mar quarter 1860 at Longtown and if this is the right Mary she was actually 1 year old when the census was taken. There is a death for John Johnston registered at Longtown in Oct-Nov-Dec 1859. Can't find a marriage, though.