I'm a youngster (only in this forum can I say that) at 58. 2 years ago I took a 3 week vacation at about this time of year. I had done multiple centuries before hand and had a century scheduled the week after I got back. During the vacation I did a ton of walking, and 2 very short easy bike rides. The century after coming back was also relatively flat - 4000 feet, with one large hill - a 2400 foot climb over 5 miles. I cramped horribly on that climb, tried to continue and ended up sagging out at the 70 mile mark. In looking back, if I had gone out a little slower and taken a few breaks on the hill I would have been fine, so my take away is, you should be fine, just don't push it hard at the beginning, save some for later in the ride.