I recently ran a Spartan race for the first time because a friend talked me into it. I was a runner in college, but haven't run a race in 15 years. But I do jog maybe 2 to 3 times a week for 30 minutes. Other than the jogging my race prep consisted of watching a YouTube video of the obstacles the night before. I completed all but 2 of the obstacles (tire flip and the climbing wall traverse like one) and almost failed a third (rope climb). I don't think I could flip the tire without a lot of training but the rest were very doable (but do wear you out).
Anyway, my opinion is that it's a footrace with obstacles and being a good runner is 95% of what it takes. Maybe you won't win, but you'll finish very high by just being a great runner. There weren't any 1530 5k runners at the event I was at.