You have a choice:
- Get your tail hung in your chainwheel trying to wrap your brain around all the inexplicably contradicting advice of "serious" cyclists
- Go out and get yourself a cheap hardtail BSO MTB and actually start cycling some before you blow a bunch of money on something you're not yet sure you like.
This is really cheap and provides the same exercise as any other bike recommendation, but won't run more than $100 or so (or whatever your currency is -- mine was Y12,000). Since your bike is cheap you can do fun stuff without financial risk, like learning how to take it apart, fitting out some cheapish clipless/push pedals (like these: http://www.amazon.co.jp/SHIMANO-%E3%82%B7%E3%83%9E%E3%83%8E-PD-A530-EPDA530-%E7%89%87%E9%9D%A2SPD%E3%83%9A%E3%83%80%E3%83%AB/dp/B001EIEH0G/ my favorite for souping up a city bike, cruiser or BSO), new saddle styles and seatpost lengths, or whatever.
You'll find out about chains wearing out, the difference between a comfortable tire pressure and a fast tire pressure, how hot breaks can get, how strong your legs can get from sprinting up short steep hills, how cramped/comfortable different seat/handlebar configurations can be, and a million other weird things you just can't know about cycling until you've spent a few hundred hours in the saddle. All without blowing a small fortune to satisfy the vanity of the "real" cyclists who seem to constantly forget that the basic feature set on the lowest MTB BSO today blows away the most extravagant features available on professional bikes in the 80's.
MTBs also have good gearing for heavy folks to start on -- until you become no-so-heavy. You don't need shocks and all the cool gizmos on newer MTBs if you're going to be on the road trying to get your heart rate up, though -- so give the shiny new stuff with the $4,000 pricetags a pass.
For those who say a cheap bike won't last long, I'm proof that this is simply not true -- I've got 3 BSOs, two of which are nearing 10,000km, all of which are demonstrating vastly superior drivetrain durability than my (enormously) more expensive road and crossbikes. Most of the price difference is defined in terms of weight, not features (except in extreme cases). Just about everything on a bike is wear-outable anyway, so as bits break either upgrade old components (and learn a huge amount about how bikes work in the process) or buy new bikes if your budget can afford it (and still learn a lot).