A trail that is challenging and is nothing but a steady incline is Barr Trail going up Pikes Peak in Manitou Springs, CO (Colorado Springs, CO).  I found it a fun and enjoyable hike, however the last two miles at the top of of the trail (Pikes Peak) are tuff.  I laugh everytime I think about my hike up the mountain, becasue while I was hiking up a few people were running up it.  As I learned they were training for the Pikes Peak Marathon, so I would put them in the professional catagory.