GK is the man. And GK has competed against Litton in many, many races but I don't think he's ever seen the guy.
Now I don't know how Litton does it exactly, but perhaps one of the best indicators is from pictures like this where he is in last place wearing a black raincoat at Deadwood SouthDakota Trail Marathon looking around for race staff before he crosses the starting line:
http://2.bp.blogspot.com/-qkyZ37CESrM/Tot_8xd-wfI/AAAAAAAAAS8/brxkrN2tv74/s1600/Screen+shot+2011-10-04+at+5.47.32+PM.png
My best guess (and a guide for potential marathon cheating)
Step 1-Research course and where the mats are going to be. (this info is often given in race info). And Make sure it's a course where you can off-road it without raising suspicions. e.g. Boston would be a good course where you can pretend to be a spectator and take transportation whereas some race in the desert might not be. Note: This is very important, and Litton has made mistakes doing this (e.g. at Go St Louis Marathon there were mats at 6mi, Halfway(13.1), and 20mi and Litton registered at the more obviously monitored Halfway13.1 mat but missed less obviously monitored 6mi &20mi mats.)
Step 2-Line up in back of the starting pack dressed in clothing that makes it look like you're a fat out of shape walker/spectator. Caps, Sunglasses, Baggy windbreakers/hoodies, & baggy sweats work best
Step 3-Wait about 2-4mins until most racers have tipped the starting line. Then once you cross, start your watch.
http://2.bp.blogspot.com/-qkyZ37CESrM/Tot_8xd-wfI/AAAAAAAAAS8/brxkrN2tv74/s1600/Screen+shot+2011-10-04+at+5.47.32+PM.png
Step 4- Continue jogging like you have a load in your pants for a good 1/2mile to 2miles (at least enough to get any suspicious race staff off your tail) then veer off course by either looking behind you first or faking some kind of injury/bathroom break.
Step 5-Drive/Take Public Transportation/Taxi to the next mat. (depends on how the course is set up....)
Step 6 - walk close enough to next mat pretending to be a spectator just to tip it off (As Ouch1000 alluded to in previous post, the mats are VERY sensitive - It's not hard to get close enough to them to register them) hardest part -> very important: MAKE SURE YOU DONT MISCALCULATE your time AND ARRIVE TOO SOON!!!
Repeat Step 6 for all other checkpoint locations given in race information. (hardest part->very important: MAKE SURE YOU DONT MISCALCULATE your time AND ARRIVE TOO SOON!!!)
Step 7 -before you take transportation and arrive at last checkpoint before the finish, strip your spectator outfit so your racing outfit and bib are visible.
Step 8 - MOST DIFFICULT STEP: jump back on the course about 1/4-2 miles before the last checkpoint mat (last mat prior to the finish mat). This needs to be well planned. Obviously You need to do this in a way so nobody really notices/cares
Step 9 - CONGRATULATIONS!!! YOUR HARDEST WORK IS DONE. JOG TO THE FINISH BUT MAKE SURE YOU DIDNT MISCALCULATE your time AND ARRIVE AT THE FINISH TOO SOON!!!! AND as you cross the finish, look really tired and Try not to WALK TOO MUCH!!!!