... choose life as an end in itself?
The question appears to be equating two ideas that are not equivalent. What is open to man's choice is (a) to live or not, and, as a consequence, (b) what code of values to live by, which (c) begins with a standard of value, i.e., an ultimate value, i.e., an end in itself. Life does not become an end in itself because someone chooses it. Life already is an end in itself. Man's only choice is to recognize that fact and act accordingly, or not.
The topic of "Ultimate Value" in The Ayn Rand Lexicon includes Ayn Rand's most definitive statement on the meaning of "end in itself" in relation to the choice to live:
Without an ultimate goal or end, there can be no lesser goals or means: a series of means going off into an infinite progression toward a nonexistent end is a metaphysical and epistemological impossibility. It is only an ultimate goal, an end in itself, that makes the existence of values possible. Metaphysically, life is the only phenomenon that is an end in itself: a value gained and kept by a constant process of action.
This excerpt goes on to reiterate the epistemological dependence of "value" on "life."
Life as an end in itself simply refers to (integrates) the fact that (a) living entities perform goal-directed actions, and (b) maintaining and strengthening the capacity for living action depends on such actions. This much is not open to man's choice, except to recognize it or not (and to recognize its implications for man's life, or not).
Update: Infinite Regress
In the comments, the questioner asks about Ayn Rand's statement that "a series of means going off into an infinite progression toward a nonexistent end is a metaphysical and epistemological impossibility":
I don't get why it's a "metaphysical and epistemological impossibility". Is she making a point about nothing being actually infinite? What I do understand is that without life no values or pursuits are possible and to me that seems like the only justification necessary for calling it the ultimate end.
If I understand the TOE excerpt by Ayn Rand correctly, the first sentence is the thesis, and the rest of the paragraph is the elaboration of the thesis. Ayn Rand first elaborates on metaphysical impossibility, as excerpted above. The same paragraph, not excerpted above, then goes on to elaborate on epistemological impossibility. For the full paragraph, refer to the topic of "Ultimate Value" in The Ayn Rand Lexicon.
It may also be instructive to remember that a process of identifying and integrating the material provided by man's senses (i.e., a process of reason) needs to integrate all relevant evidence and issues, including issues of infinite regress of means and ends. (How can a final "end" be said to exist if it isn't a final end, but only a means to something further?) One may question whether the infinite regress issue should be presented where Ayn Rand presents it, or introduced later in the development. I do not know all the considerations that Ayn Rand undoubtedly had in mind in organizing her writing as she did, but I can guess that she may have been seeking in that article to emphasize issues that academic philosophers often raise.