Since you have some comfort issues, it may be a better idea to get a bike fit before buying a bike if that's within your budget. For example, if you're planning on spending $1,500 on the bike, spend $300 of it on a decent bike fit up front, then spend the $1,000 on the bike, spend the $200 on saddle, stem, and bars; it'll fit better, you'll be more comfortable, and therefore you'll ride it more than an uncomfortable $1,500 bike. If your budget is below $700, then it might be better to get the bike, then get a bike fit down the road. Most bike fitters will help you select a bike that fits your needs.
For fitness/workout purposes, I would lean towards a nice and light fixed gear/single speed bike. Seems like you're not an avid cyclist yet and the simplicity of a single speed can help in getting you out and riding more than a geared bike. Some beginners don't feel comfortable shifting. I've seen many of my family members refuse to shift to an easier gear up a hill many times. If you're already familiar with shifting, adjusting, and maintaining your geared bike, then no reason to not go with a geared bike.
I don't know how hilly your area is, but I'm in a relatively flat area(500 ft elevation gain over 20 miles). I loved riding my fixed gear(sold this year to fund a new road bike), and I will probably get another one in the next few years.