In reply to afshapes:
Historically multi-day running races have been based on running as far as possible each day, rather than a fixed distance as fast as possible (e.g. Spine race, Sydney-Melbourne Ultra).
I think if you were going for 3 weeks then it would be around a marathon per day, though people are capable of going further than that for many days*. Consider that casual Sunday cyclists often cycle for many hours, while very few runners go for more than a couple.
I would consider a 100 mile cycle race to be the equivalent to a marathon, and this year's Tour de France stages are on average a little over 100 miles. Of course they also include pretty serious amounts of climb so you would have to include that.
I wonder if the tactics would be similar?
*https://en.wikipedia.org/wiki/Self-Transcendence_3100_Mile_Race