My lads furthest ride had been 43 miles with me earlier this year. A month or so later he managed103 miles.
This was done with me, 2 uncles, his cousins husband.
We looked after him, reining him back when excited and cajoling him along when it got tough. The route was fairly flat, bits of undulation through Suffolk. The period between 70- 95 miles were into a stern 20mph plus headwind along straight fen roads. We sat him at the back and the 4 of us took turns riding at a steady 12-13mph on the front. Once back into Cambridge he sprinted and left us for dead, the move of a seasoned pro!
See if there any CC informal rides in your area....
Join a club
Ride alone 20 miles to your west (if possible) have tea and cake, then return on the tail wind.
Don't go for speed, just go steady.
Eat and drink well, including the day before!