For reference from another 40-something: my husband and I just got back from a 3-day Grand Canyon hike. We went to Bright Angel Campground at the bottom near the river from the South Kaibab trail in about 8 hours, stayed one night, went up to the Indian Garden campground the next day in about 5 hours, stayed one night, and then out where you end up at the Bright Angel trailhead the next day in about 5 hours. We were pretty spent.
Our calves are still a little sore (after 9 days)! This is mainly from going to the bottom in one day. We did not do as much training as we should have. We are not overweight and have been on several other treking adventures. In general, we do some sort of 30 min aerobic workout 2-3 times a week, but doing some more hiking before the trip especially with our full packs would have benefited us.
One thing to consider when comparing these stories is that a day hiker will not be carrying nearly as much of a load with them as a backpacker. A tent, sleeping bags, food, clothes, cooking gear, etc. will have a big effect on what your muscles need to accomplish.
We saw all ages on the trail and were amazed at how fit some of the older people were. We did not see a lot of overweight people (except riding the mules!) Definitely plan on bringing a lot of water, snacks and sunscreen. We always hike with hiking poles as it helps a lot, especially for knee joints on the downhill. Hiking boots, synthetic clothing, and layering are also a must. A hat or sun visor with a brim is recommended.