I have yet to do a solo unsupported century. But I've done many 80+ mile solo rides. I don't train with a power meter, or even heart rate monitor to gauge my effort. But I usually knock out 80 milers in around 5 hours. I usually give myself a time limit to reach home by so have enough daylight to spend with my kid. I adjust my effort and pace to meet that time goal. Last thing I want to do is be riding alone for a whole afternoon while missing precious family time.