echappist wrote:
not to throw cold water on this, but think it through a bit.
I bought a new MTB ($3700 all said and done), because I no longer felt safe riding on the roads, but could ride on the local trails. To say that it's eye-opening is a bit of an understatement. There was a thread here asking about relative safety. For that, be prepared to fall or crash. Already happened 3 times since I bought the bike in May.
Be ready to spend tons of time (and perhaps some money) learning how to ride properly. I'm still scared stiff (arms literally stiff from grabbing onto the bars for dear life) doing switchback descents, but it's getting better. Also don't expect your MTB workout to have much value in calorie-burning, at least not in the beginning.
This is true. A friend of mine, a good longtime road cyclist, has been getting into mountain biking by riding with me. We ride natural singletrack that is mostly used for hiking. It has taken me back and has been amusing as hell, if only for the swearing. It really emphasized that unless you grew up riding street on a BMX it doesn't matter how good of road cyclist you are or how long you have been riding, you will ride like a muppet on real trail that is minimally technical.
Unless the OP has no intention of ever riding something other than fire roads, gravel, or smooth and buffed singletrack, I would suggest a FS rather than a hardtail. A mid travel FS (about 120mm these days) is a do-it-all bike that will work for what most people coming from an aerobic endurance background will be doing. Step down to 100mm on both ends if XC and marathon racing is a possibility and a less general purpose MTB is not a concern. You have to keep in mind the whole MTB marketing machine is targeted toward endurobros. These guys in their motocross costumes value their worth as a mountain biker based on how fast they can descend through gnar, how far they jump, and how big of a drop they can do. They and the industry along with them are always pushing for more travel. Talk to a shop rat and they'll give you some bullshit about how well their 150mm ride climbs. There is a real tendency to put people on more bike than they need.