Steamer makes excellent points about pace and ftp. Another consideration is temperature. Cooler temperatures usually allows a harder effort. For example I planned to do one 400k all out earlier this year meaning an optimized ride for time. The first four hours were cool and I rode at 220-230 watts and a HR in the mid 130's which is 6-7 beats higher than LT1 or AT. In the heat of the day I dialed it back to 160-170 watts and a HR of around 110-115. If I tried to maintain the earlier pace I would have over heated. Few Randos ride with power meters or know their lactate thresholds but it is not really needed. If you can't talk easily the pace is too fast for a 400k for almost all. On. 200k? Let it rip......live and learn