I'm around 300 lbs + 15 or more lbs of gear, and long distance rides works best for me. Started with 1-2 hours rides, with an average speeds around 9-10 mph. Now, after 3 years of my cycling fever, my rides on my hybrid are anywhere between 1.5 hrs to 12 hrs. Average speed anywhere between 13 and 18 mph.
I also hate hills, and try to conquer them as fast as possible, and that's why I'm always first one on the top. I bet that helps with my overall shape lol.
Cycling should be fun. Don't worry about average speeds or distance too much. Have fun and see what works the best for you. Find your sweet spot - best gearing and speed, and enjoy your ride!