First long ride (~100 km) from London to Brighton after getting the 100GBP Argos "crusader" bike. We tried following a basic Google mapped route but weren't used to the roundabouts. Almost ended up on an M series highway for one of the turns. And I learned the importance of continuously eating on a long distance bike ride. Felt a bit faint before Ditchling; blood sugar was low.
Update 16/08/2015: I've done several more London to Brighton rides, each better prepared than the last. Different routes and on different bikes. Still very enjoyable, it is a classic ride with the London Beer and Bike Group.