Should definitely be rated as strenuous. Mostly steep ups and downs with scattered, lose rocks. Only a few sections have rock stairs. Also my tracker said it was 13.7 miles not 11.9. Regardless of its challenge, the Skyline Trail is the most beautiful and exciting way to experience Blue Hills . It traverses all the best hills, most have views and some have open panoramic views. On a clear day you can see Boston and the ocean. Very easy trail to follow, just look for the blue blazes. Certainly worth the effort and I would definitely hike it again.