I have gone through three different bikes in the last five or six years before finding an extremely comfortable one.
I found that straight flat bars start killing my neck and lower back after more than 20 minutes of riding. Medium swept back bars work the best and the reach from the seat and the height is critical.
I love my Sixthreezero EVRYjourney step-thru. The stock swept back bars were a little much, so I replaced them with a classic style Wald handlebar. Pivoted the seat mount 180 degrees to move the seat forward about 2 inches. Choose the heavier steel frame because I find aluminum frames are just harsher riding. The obsession with weight is absolutely stupid unless you are competing against Lance Armstrong. The only time I ever notice a bike is heavier is if I pick it up to carry it.
I can ride that bike for hours without any discomfort whatsoever. Have a basket on the back for utility, so a step-thru just makes sense instead of trying to swing a leg over super high. I find seven or eight speeds are more than enough for all around terrain including hills, allowing for very efficient faster paced cruising.
You can get excellent cardio on a comfort bike, you don't need a road bike. The only difference is you average 2 or 3 miles per hour faster on a road bike vs a comfort bike when expending the same energy.
I will never understand why people ride a road bike for casual riding or cardio work outs. They are just plain uncomfortable and not as versatile. Their only logical use is for competition. And no, I wouldn't be caught dead riding in lycra tights.
Choosing a proper comfortable bike gets you out there riding and enjoying every minute of it. It doesn't feel like a dedicated work out, it feels like something fun you want to do