If you're not used to the way they feel, go into a bike store and get them to size you up on a road bike. You sit differently on them, and it's very important to get the top tube length right for your torso size to avoid back pain on long rides. Once you know what size top tube to be looking for, you can check all the major manufacturers for information about their frame sizes. Unfortunately they all have slightly different geometries, so it will take a while to narrow this down.
For long rides I much prefer a steel frame on a road bike. . . it might be a lb or two heavier, but it absorbs bumps and is much nicer on your ass and hands over the long haul. A 300$ budget wouldn't take you far around here looking for a road bike . . . but prices can vary from place to place.
There are many good bike brands . . . look for Surly, Specialized, Giant, Jamis, etc.