I am now nearing age 64 and have had the good fortune to have hiked in Glacier NP for 6 years. Have not done the route you propose, just a few pieces of it.
First, I heartily endorse the comments made by Sam Haraldson. Second, while I am in good shape for my age, my desire now is to limit my trips with a pack to 8 - 10 miles. I know that I could do 17 to 20 miles a day; it is just that I choose not to as of now.
Having said that, if you keep to your route, on the first part, remember that you have got a good 17+ mile hike to get from the Kintla Lake campground trailhead to the Hole-in-the-Wall camp. (You might double check your mileage on that because I think it is closer to 20 miles....) Be sure to leave early enough for that great hike! Also, note that Upper Kintla Lake is listed on my topo as 4,371 ft. of elevation; Boulder Pass is in the 7,000 area. That means you have got around 2,600 ft. of elevation gain. In other words, your hiking miles per hour is probably going to drop compared to the trail around Kintla Lake.
For the Granite Park hike to Logan Pass, it is about 8 wonderful miles. With packs, it took us about 6 hours to hike from Logan Pass to the Granite park chalet, but we stopped to take pics, enjoy the views, eat lunch, chat with other hikers, etc. We saw our first wolverine near Haystack Butte! After your shuttle, you have a long, but good 10 mile hike from REY to ELL. Similar to my comments about the Kintla-HOL hike, allow yourself plenty of time.
Lastly, early September is my favorite time to be in the park. Less people; fewer bugs; etc. But, in your planning, know that it can snow at any time there. Particularly in September. When we have hiked there at that time, we dress and carry gear in case the temps drop to 15 - 20 degrees F. Moreover, when there is a good bit of snow on the ground, our hiking speed dropped considerably.
Hope these notes from an old geezer help. More importantly I hope that your trip is one of those once-in-a-lifetime treks! To me Glacier is a magical, wonderful place. I think it is the crown jewel of the entire US.