Very popular among runners, joggers and in-line skaters is the paved BCP (Boulder Creek Path) which winds through the middle of the city and leads then towards the mountains.
Once you run out of city limits you'll pass the freeclimbing training rocks and later the Boulder Falls.
An outstanding alternative form North Boulder's trails:
The Hogsback Mountain is an ideal distance for newcomer runners: coming from the city drive either Broadway or US 36 north and turn left into Lee Hill Rd. Shortly before you enter the foothills you'll find parking possibilities at the trail head of "Wonderland Lake". From this parking lot run north and follow the trail. After a couple of hundred meters you can choose the small path winding in western direction up to Hogsback. It gets real steep so be prepared for the thin air.
Not far away is Boulder Valley Ranch located: Here you can run in a more or less flat environment but in higher elevation than Boulder City. It's a perfect training area for long distance runs, because you can run from about 5 km (3 mi) up to marathon distance when you combine the ranch's loop trail with a trail that connect to the loop at half way (above the small pond) in eastern direction. Another trail connect on the western side of the loop to wards the foothills. Boulder Valley Ranch is especially nice because you're often in company of horses and cattle.
Also professionals use Boulder Valley Ranch for training purposes.
On most trails you'll probably get the right motivation when you take a look at the fantastic mountain scenery.