If you want to earn some bragging rights or catch a glimpse of runners in serious training, take a run on Magnolia Rd. It’s reputed to be the steepest paved and maintained road in the United States. The road runs from Boulder Canyon to Nederland for 13 miles, climbing 3,500 feet. The steepest part is the first 4-5 miles, which features grades of up to 17%! Just short of the 5 mile mark, at the intersection with Beach Rd., Magnolia Rd. turns from paved to dirt…and becomes flatter/more rolling. So you could park here and do a ~15 mile out and back, which is an 1,895 foot elevation gain.
This is a good spot to do some serious hill training. Traffic is minimal. The scenery is a combination of wooded and open, with occasional mountain views. There are also some homes along here.
The closest section to Boulder starts at ~7 miles west of the city off Boulder Canyon Dr. (Rt. 119), and heads to Nederland, where it actually runs back into Rt. 119 (Peak-to-Peak Highway) and continues another 1.5 miles to an unceremonious end.