Ishmael could just be his subconscious telling him to do things. Like muscle reactions, saying "wake the fuck up" or "move your ass" and Snake is battling the effects of waking up from a 9 year coma so his military brain kicks in and helps him survive. It is crazy but a few things don't sit right
-He has no face, and none of the main characters do up until Ocelot in that sequence. Demonic Volgin, Psycho Mantis, Jack Baur/Ishmael. They're all deliberately hidden.
-Ishmael never helps him move faster, he rolls him over a few times and injects him or whatever but when he says they have to move and get out fast, he just waits for snake to crawl around and never supports him
- finally he disappears after the Ambulance crash. he's suddenly gone and not on the horse with Ocelot and Snake. He's obviously an important character cos he has Kiefer Sutherland voicing him and he has been watching snake for 9 years, so he's not dead that fast