The North Loop takes a counter-clockwise route from the parking lot, following trail junctions 9 to 1. It's a fun trail that gets a little narrow in places (really, watch that tree in the picture) and is mostly hard pack. There are a few spots where its gets kinda "chunky" and a few short spots where it turns to Talcum powder/Pea-gravel over hard pack (not so nice...). Those spots tend to keep you on your toes though, so its all good!
A nice climb at the beginning gets the lungs and legs working, but some of the switchbacks are kinda tight. There's a nice little overview of the city at the top of the climb, so its worth stopping and checking out. From here there's a touch more climbing to do (shallow grade), but once you reach Junction 5, there's a nice bit of flow till you reach the Sierra Del Norte Road Crossing.
Another bit of climbing up to Junction 3, nothing major though nice and easy (switchbacks have a good radius). You climb a bit more after Junction 3, but its nothing like the First Climb or the Climb up to 3, and after that it's just a fun time to be had back to the parking lot.