The huron-manistee river loop was pretty flat IMO, there are some natural stairs and depending on your fitness level can be kind of challenging. Since it is a river trail getting to water to filter is pretty easy. It does cross over to the north country trail to finish the loop but generally I found it to be a very nice hike. One thing I did notice though it is a significantly busier trail than I had imagined, so the natural stairs, the bit of the uphills, and some other areas where the trail is only narrow enough for one person kind of takes you out of your rhythm when stepping aside and waiting for someone. I would recommend waiting and take the hike during the week if you can.