I agree with the renting idea. It is REALLY hard to know what you want when you don't know what you want. I would suggest renting more than once and more than one type of bike if that is possible. That way you get to see how different bikes feel. Concentrate on how each one feels, what you like or don't like and how it could be better. The more you know about what you like then the easier it will be to find a bike that will make you happy long-term.
Most of all, just get out there and start pedaling! I wasted a lot of years because I was not a 175 pound pro-level body type. I just ASSUMED that I couldn't.
What a crock!
Like the Nike ad said, "Just do it!"