IME the interval for needed breaks gets shorter as you go along, so I try to do something close to half of longer rides at a throw, Then take a decent meal break (don't overeat), leaving more than half of the planned time for the 2nd half, probably with 1 or 2 short breaks along the way.
On a typical century I'll do the first 50 miles or so in one third of my planned time, give some back at the break, but still have plenty of time for the second half. Also, schedule your ride with plenty of open time after the scheduled finish. IME rides where things get easier on the 2nd half of a ride are truly the exception rather than the rule.
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