I'm not sure where they are running the race at in Wickenburg but I have ridden at Alto Pit and a trail up to Vulture mine. Exactly what you would expect from Arizona, rocks, dust and Cholla cactus. You can check ArizonaTrailriders.com and RidingArizona.com for more local info.
Don't get carried away with the "warmth". It will most likely be in the lower 70's [during the day] when you are there but Wickenburg is over 2000ft in elevation and far outside of the heat of Phoenix. It WILL be cool to cold at night. I know 30's doesn't sound cold to you [I'm from New Buffalo, MI myself ] but it is when you were planning on 85 degrees, sunscreen and Margaritas and you forget pants, sweatswirt and a jacket.