Extending line by specified distance in ArcGIS for Desktop?

I have a purely aesthetic layer which has arrow symbols. Some aren't showing up right because the line is too small. I have selected maybe 50 records where I need to extend this line by a given number (ex. 2 meters). The extend line tool only extends lines to a specified intersection, so this tool isn't what I'm looking for.

I've tried editing the shape length field but it won't let me. Is there a simple way of doing this via field calculator or within the editor tool bar?

Answer

Well I think I've gotten it down for lines of any vertex count. I haven't attempted multipart lines since I've never messed with it in arcpy. The coding was made a bit more difficult since there isn't write access to the lastPoint property for Geometry objects. Instead of using the slope (which was my initial thought), I used the code from this SO question. It doesn't rely on trigonometry, so it should be slightly more efficient. The following code works by moving the endpoint of a line to a new coordinate that lies along the prolongation of a line from the last two vertices. I tested it on a shapefile.

from math import hypot
import collections
from operator import add
import arcpy

layer = arcpy.GetParameterAsText(0)
distance = float(arcpy.GetParameterAsText(1))


#Computes new coordinates x3,y3 at a specified distance
#along the prolongation of the line from x1,y1 to x2,y2
def newcoord(coords, dist):
    (x1,y1),(x2,y2) = coords
    dx = x2 - x1
    dy = y2 - y1
    linelen = hypot(dx, dy)

    x3 = x2 + dx/linelen * dist

    y3 = y2 + dy/linelen * dist    
    return x3, y3

#accumulate([1,2,3,4,5]) --> 1 3 6 10 15
#Equivalent to itertools.accumulate() which isn't present in Python 2.7
def accumulate(iterable):    
    it = iter(iterable)
    total = next(it)
    yield total
    for element in it:

        total = add(total, element)
        yield total

#OID is needed to determine how to break up flat list of data by feature.
coordinates = [[row[0], row[1]] for row in
               arcpy.da.SearchCursor(layer, ["OID@", "SHAPE@XY"], explode_to_points=True)]

oid,vert = zip(*coordinates)

#Construct list of numbers that mark the start of a new feature class.

#This is created by counting OIDS and then accumulating the values.
vertcounts = list(accumulate(collections.Counter(oid).values()))

#Grab the last two vertices of each feature
lastpoint = [point for x,point in enumerate(vert) if x+1 in vertcounts or x+2 in vertcounts]

#Convert flat list of tuples to list of lists of tuples.
#Obtain list of tuples of new end coordinates.
newvert = [newcoord(y, distance) for y in zip(*[iter(lastpoint)]*2)]    


j = 0
with arcpy.da.UpdateCursor(layer, "SHAPE@XY", explode_to_points=True) as rows:
    for i,row in enumerate(rows):
        if i+1 in vertcounts:            
            row[0] = newvert[j]
            j+=1
            rows.updateRow(row)

I set the symbology to arrow at end for categories based on OID so that it will be easier to see the separation between features. Labeling was set to count vertices.