Very nice day hike!
I would rate it as moderate (rather than difficult), but still very enjoyable for experienced hikers. The trail doesn't go steep except for the very last ascend to McNeil Point, which is pretty short. I was also lucky with the weather - not too hot, not too cold (in the 60's), which made it a great day to enjoy. View from McNeil Point and along the way was quite nice as well. Overall time: 4:40h, including ~40mins dedicated to eating blueberries.
One correction: the map from AllTrails app was very useful to track progress along, but it only shows the trail up to and around the loop, without the leg to the McNeil Point. There is about one mile more from the far end of the loop to reach the McNeil Point, which is a nice small peak under Mt. Hood (well worth the extra mile, so do budget for it).
This is a nice and pretty popular trail. There are always more than a few backpackers camping out. It's a nice gradual climb all the way up, I wouldn't say there are extreme incline sections. Always best on a nice clear day to see all the surrounding mountains. If you have time make sure you continue past the shelter and head over to the glacier.
I hiked this trail almost 2 years ago in August and it has been one of my favorite hikes ever since. I got lost at the end though and never made it to the little fort that the trail leads to, as the trail splits in half towards the very end. I instead ended up hiking about a mile further than I needed to to some rocky point that was pretty steep (maybe even a better ending to the hike though!) I would say that this trail is only difficult because of the distance and the fact that it is uphill the whole time.