Legs feeling heavy means you are using the full capability of certain muscle groups for long periods of time. Humans evolved an ability to outlast predators and tire out prey. Today we use this ability for cycling & long distance jogging. My advice is to keep hydrated and don't push too hard as you ease into this new set of activities. Over a few months the muscles involved will get stronger and you will find yourself less and less challenged, and your legs will only feel tired when you
really push yourself.
Eventually if you keep riding enough and diet etc is not crazy, you will find yourself able to ride huge distances with not much more than a short break to let your blood become more oxegenated & wastes from your muscles cleared out. This is why hydration is important, gotta keep the wastes from muscles working constantly draining so the liver, kidneys & sweat glands can filter it out.
Good luck & keep up the good work!
- Andy