Some trails are difficult. Depends on your physical ability, stamina, endurance, etc.
I'm nearly 30, and not in the best of shape (is round a shape?
), but with a carefully-weighed out pack (27 lbs.), lots of water, hiking poles, and no big rush to kill yourself each day, it was fine.
I went with a group of two others, who were in much better shape than I, and we all stayed at the same pace, stopping when we needed to for water, quick rests, photo ops, etc. At the end of the trail, they said I didn't slow down or speed up their usual pace at all.
After my second trip to I.R., I would probably change most of the hiking intensity levels in the guidebooks that say "easy" to "moderate". There wasn't any big "break-your-neck" type adventures that I've come across out there. The biggest challenges for me were not distance, but elevation (the "ups" were worse than the "downs"). I would say that my biggest challenges this year were going "uphill" from Windigo to Island Mine (most do it backwards coming from Rock Harbor, and have it as a downhill leg into Windigo). Another minor "toughie" was the West Hugginin Loop going from Minong up to Superior--less travelled and super "rooty" and bouldery in some spots.
I could safely lose 80-100 lbs., and I still did IR just fine.
