In a 2010 interview, Armstrong explained that NASA limited the first moonwalk's time and distance because there was no empirical proof of how much cooling water the astronauts' PLSS backpacks would consume to handle their body heat generation while working on the Moon.
ɪn ɐ tˈuː θˈaʊzənd tˈɛn ˈɪntɚvjˌuː, ˈɑːɹmstɹɔŋ ɛksplˈeɪnd ðæt nˈæsɐ lˈɪmɪɾᵻd ðə fˈɜːst mˈuːnwɔːks tˈaɪm ænd dˈɪstəns bɪkˈʌz ðɛɹwˌʌz nˈoʊ ɛmpˈɪɹɪkəl pɹˈuːf ʌv hˌaʊ mˈʌtʃ kˈuːlɪŋ wˈɔːɾɚ ðɪ ˈæstɹənˌɔːts pˌiːˌɛlˌɛsˈɛs bˈækpæks wʊd kənsˈuːm tə hˈændəl ðɛɹ bˈɑːdi hˈiːt dʒˌɛnɚɹˈeɪʃən wˌaɪl wˈɜːkɪŋ ɔnðə mˈuːn.