Allen fell 150 feet off an exposed ridge during Norway's Tromsø Skyrace, and is now on the road to recovery.
Amid one of the deepest fields in trail-ultra history, Americans had a strong showing.
The two American runners both claimed victory in a race typically dominated by Europeans.
The two semi-pro road racers have been dabbling in the trails for a few years.
The jack-of-all-distances finished 10th in the Leadville 100, and then seven hours later ran the Pikes Peak Marathon.
Walmsley broke the overall record by three minutes, and Flynn broke the women's record by 20.
The stories behind some of the craziest moments at this year's Hardrock 100.
The two runners covered 60 miles with 22,600 feet of elevation gain in 32 hours 50 minutes.
The Harvey Manning Peak Challenge saw its first finishers, Ras Vaughan and Seth Wolpin. It took them 78 hours 36 minutes.
Hamilton added an extra summit to the classic Colorado peak challenge Nolan's 14, and then cruised the regular route in less than 54 hours.