Hoosier Pass (11,500' and change above sea level) from the center of Breckenridge (about 9,600' above sea level) is about 10 mi with a bit under 2,000' of climbing. Just a bit under a 3.6% average grade. Ruling grade looks to be about 7.2% near he top.
But the oxygen level can do a number on you. I took a day off at the HI Hostel in town and could really feel it.