Honestly, I would say a Surley LHT would be perfect for you, and that is not because I ride one, but it helps. If you ride a size 54 or smaller, it comes with 26 inch wheels, like your mountain bike. If you ride a larger size, it comes with 700c wheels, and you can put 700c by 45 tires on it, or 40's with fenders. So the big tire thing is covered. Front and rear rack mounts, long wheel base and a steel frame to soak up bumps. Solid 9-speed rear and triple front chain rings for climbing and durability. Drop handle bars give you the speed option, but plenty of hand positions for just riding around. Also, it has bomb proof components. From someone who has broken a bottom bracket and twisted up a chain like Christmas tree garland, I can relate to the "unusually strong legs" remarks. I got accused of having calf implants this past Sunday.
Just my thoughts. A lot depends on your budget. Good luck, and let us know what you decide.
D