It's also not "slow", but "steady" at a 60-70% MHR pace. That's actually fairly fast where you can barely hold a conversation. But at least that's sustainable for 3-5 hours at a time. The idea is to develop pedaling form, build up ligaments and tendons, and capillaries in the muscles in preparation for real workouts later. You'll also tax the energy-system and get better at digesting and delivering on-the-bike meals as well as tapping into your fat stores.