My 100 miler day rides have been in Kent/Surrey or France. That particular one had a peak elevation of 290m from the coast, plus a number of other climbs. I had done one in Alsace of 100 miles in 8.25 hrs which had much more altitude gain and three quite steep climbs. Checking my cycle-camping records, I have covered nearly 8500 miles on these trips, by Brompton, during the last twelve years.