Find a used road bike on CL. Just make sure it is WSD (women specific design). The geometry of the bike is built for ladies. But honestly, in terms of fitness, I'm so happy I bought a semi-nice bike. The $550 was worth every penny this summer, and for many more to come! The lightness over a crossover/hybrid bike makes for a better workout in my opinion==you just don't get discouraged.
The best part is the shifting mechanism compared to a cheapy walmart mountain bike, although I'm sure top end hybrids are very similar. I love my sti shifters
Truth!
I paid over $400 for my Trek 7.2FX a few years ago, and it's labeled as a "performance hybrid". Super light, and shifts awesome! We got my wife a $80 Huffy from Target, and it didn't last 3 towpath rides before it wouldn't shift for s***. lol