Increment Alphanumeric Value of a Cell on each print job

hello I need help with the following macro, it works great but only with numeric values but it doesn't work with alphanumeric values.

Sub PrintJobs()
Dim i As Long, startnum As Long, lastnum As Long

    startnum = Application.InputBox("First Order", "Print Job Number", 1, , , , , 1)
    lastnum = Application.InputBox("Last Order", "Print Job Number", 1, , , , , 1)

    For i = startnum To lastnum
        Range("F7").Value = i
        ActiveWindow.SelectedSheets.PrintOut
    Next

End Sub

for example I want the following value G20141530 in F7 to increment by one after each print so if I print 5 sheets I get G20141530,G20141531,
G20141532 and so on. any help would be greatly appreciated.

thanks xladept for your help, code works great the only thing is when I cancel the printjob I get a debug error. I have underline the debug error in the code.

Sub RudyPrintJobs()
Dim i As Long, j As Integer, startnum As String, lastnum As String, Alfa As String
    startnum = Application.InputBox("First Order", "Print Job Number")
    j = 1: Do Until IsNumeric(Mid(startnum, j + 1, 1)): j = j + 1: Loop
    Alfa = Left(startnum, j)
    lastnum = Application.InputBox("Last Order", "Print Job Number")
    If Left(lastnum, j) = Alfa Then
        For i = Right(startnum, Len(startnum) - j) To Right(lastnum, Len(lastnum) - j)
            Range("F7").Value = Alfa & i
            ActiveWindow.SelectedSheets.PrintOut
        Next
    End If
End Sub