Not sure where the Sheraton is relative to the HI but... If you have a map, I can say from vague memory that it takes approx 30 minutes to walk from HI to the old town (perhaps a bit longer at night when it makes sense not to walk through the memorial park just outside the old town). This should give you an idea of approx walking time.

If this is your first visit then the bits of the city I walked through were safe enough but Taxis are inexpensive if a place looks a bit dodgy.