SQL Performance with Distinct and Count

I have a stored procedure which contain so many SET statements. That is taking so long to execute. What I can do for increase the performance. One statement I have included here.

SET @VisitedOutlets=(select count (distinct CustomerId) from dbo.VisitDetail  
                        where RouteId = @intRouteID   
                        and CONVERT(VARCHAR(10),VisitDate,111) between CONVERT(VARCHAR(10),@FromDate,111)  
                        and CONVERT(VARCHAR(10),@ToDate,111));