The hard truth is that the limit of one's initial capacity to ride long distances is not just one's stamina but also one's ability to endure the seat. The organ most challenged is not the lungs or heart nor even the legs, but the butt.
Thus far I have reached Nishkina (spelling?) on the Glenn Highway, a day short of Glenallen. Thus far I have begun to acquire the initial stages of stamina and thus am able to ride longer and further each day. And the Glenn Highway is now carrying me north from the Chugach Mountain Range which has turned south following the coast, giving me flatter country to ride through. At least that's the theory.