How can I get my Staffie to go 'walkies' ?

I got Holly, as a rescue dog 6 years ago so she would be about 8 years old now. I used to walk her every day for an hour or so, no probs. We live in Norfolk, surrounded by farming land, idyllic!! She is used to the 'crow scarers' going off (ie loud bangs) BUT, farmers with guns, in the surrounding fields, terrifies her. It has got so bad that she shakes, hyperventilates and dribbles in the darkest corner of the house the minute she hears a distant shot. She flatly refuses to go for a walk. She gets all excited when I put her harness on and is raring to go, the minute we leave the garden she sits down and trembles and refuses to go.Any ideas?