The basic requirement for a 50 km-, 71 km- or 100 km-race is a Sub 5 hour marathon with at least three month of ultramarathon-specific training, or any ultra-distance equivalent to 100 km in 15 hours.
You can find detailed information about ultramarathon training, along with 14 different training plan for 50 km-, 70 km-, 100 km and 100 miles, in my book "Das große Buch vom Ultramarathon" Volume 3.
ISBN-13-978-3-7679-1241-0
To order at any book store or at amazon-Link:
Universal Training-Plans for the Taubertal 100:
100 Mile-Training Plan:
100 km-Training-Plan:
70 km Training-Plan:
50 km Training-Plan: