That is called a Harescramble race. Simular to a enduro but different in that you all start at the same time with the engine off, and race non stop for 2-4 hours depending on what track it is.
Enduros you start with the running engine, and start in small groups, and ride sections and then stop.... you might ride a 8 mile section then get to stop for 5 minutes, then start again on a new 15 mile section, then stop for lunch break, then ride the next section and so on and so forth.
Motocross is done on a track usually in a open area ( no trees ) with a bunch of turns and jumps. MX tracks usually aren't more than a mile long if even that long.
Harescramble courses are just trails in the woods, and are usually 6-10 miles long.
Every race there is always people getting hurt. Usually it is minor scrapes and bruises, but at times someone will get seriously hurt. They have medical teams at the races to respond to a accident if need be. Last year at a race I was at, the leader in my class ran wide on a turn and went off a embankment into a creek, head first, and got a concussion. He was able to walk upstream to where some spectators were at and he passed out..... they ended up calling in a helicopter and flying him to the hospital. He is okay now, but he was messed up pretty good for a few days after that race.