Many decades ago, my husband broke his chain while out on a ride with my sister and me. He walked/velocipeded 15 miles home while we rode the rest of the 50 mile loop. Since then, we always have a chain tool with us. (It was pretty funny watching him roll backwards when it broke. He thought he had just mis-shifted and was pedaling frantically trying to get the chain to engage. I guess it didn't occur to him to use his brakes.)

By the way, whenever I hear of or see someone have a chain break on a ride, it always turns out to be a Shimano chain. My husband had one literally fall to pieces on him. He put it on, went on a 750 mile tour, and limped to the endpoint six links shorter than he started.