I had to go up to the Gunflint last weekend and pick up some stuff I had forgotten and while up there I decided to hike the Caribou Rock Trail. The trail was packed, must be fall color season. I've hiked this a few times and when people asked me what the trail was like I told them it is challenging and tiring. Then I thought for a 7 mile round trip day hike it could possibly be the hardest hike I've ever done.
So, I'm curious. What are your thoughts on the most challenging day hike you've done.

I don't really get a chance to day hike. I have had many tough days of hiking, but they are always overnight or multiple day trips. The toughest day of hiking that I have had was going from Horseshoe Lake past a full site on Pose to the EP. I was swinging my legs in front of me instead of actually hiking. :) I think about that day every time I go to the gym.

I didn't know about the Caribou Rock Trail so I had to look it up.
I found some info here

That seems like a nice trail to hike.

I also don't get to day hike as well, but my hardest day was 14.5 miles from Egge Lake to Wolf Lake on the Superior Hiking Trail. That day almost finished me, but I would do it again.

My toughest, (so far), has been Ennis Lake, by Secret and Blackstone up the Fernberg north of Ely. Much of the trail is mountain goating it with tons of rocks and roots to climb, (not walk, climb), over. It's only about 2 hours long, but is challenging footing with lots of hills. These are offset by a really cool gorge that you follow along the top of, with lots of lookout spots to sit and quietly enjoy. And, best of all very few people. TG

I've heard that the Secret Blackstone trail is nice and I've passed the trailhead going up to Moose Lake. I'll have to hike it next time.

Talusman, yes Secret/Blackstone is nice and pretty easy. The more challenging part is the Ennis Loop. I love them all. I just love being outside and being blessed with the fortune and enough good health, to enjoy the woods. TG