You’ll feel the altitude, but be ok as long as you don’t try and do your usual pace. Back off about 30 seconds per mile and you should be alright. Hills won’t kill you either.
The Barr Trail in Manitou goes up, up and up for 13 miles to the top of Pikes Peak. Start is about five miles west of downtown. Start is at about 7500’. Great views from up there. Barr Camp at mile 6 is a good turnaround.
There are plenty more that others can contribute, but for the ultimate challenge, I offer you ‘The Incline’. Nothing else like it anywhere.
http://www.stevegarufi.com/manitouincline.htm
Two thousand feet of vertical in one mile. Located at the base of Pikes Peak in Manitou, maybe five miles west of downtown. It’s brutal, but satisfying. Super view at the top. Starts at the cog railroad station in Manitou at essentially the same place as the Barr Trail mentioned above. Virtually impossible to run, but you can sort of jog/fast hike up it, if you’re in shape. Bail out point at the halfway. Run 4 miles down the Barr Trail to get back to your car.
Thanks for all this! Someone else mentioned the incline, I also want to see views so I will hike that Friday. Is it pretty safe alone? Do I need to bring pepper spray?