newb tips???
Ride as much as u can, road or mountain, mtb's work on the road too, so don't make the road bike seem like some necessity. U said u wanted to lose the gut and get in shape, well, that means pushing yourself harder and harder over time and improving, so set reasonable goals and try to meet them.
Also, if u enjoy mt biking, read about it, this is one of the few sports that u can learn about from reading, trail usage is a big responsibility as well as being a lot of fun, so find out the rules of the trail and the reasons these rules exist and then follow them. Reading also helps with things like riding tips to help overcome performance issues and to help u do things on a bike u may not be comfortable with or have trouble with. Reading can also help with maintaining your bike, or upgrading or buying a new bike. These guys at the big bike mags have been riding mtb's forever and they get to ride a handful of bikes ecvery month where as the average rider may only ride a handful of bikes in a lifetime. They can pick out different handling and performance characteristics in bikes that even the most experienced racer may not be able to. So, spend some nights on the interweb, or with yur hed in a book or mag learning about the sport.
Then there's the issue of the bike itself. Some department stores actually sell some bikes that ARE trailworthy, not Wal Mart or anything, but Dick's or something, up to the five hundred dollar range. The problem is, u don't know what crackhead put them together, or what. You're weight has a lot to do with whether or not u can get away with riding a four hundred dollar bike or not. Most bikes in general aren't made for people over 175 lbs, beyond that you're bike, or components of it may be prone to catastrophic failure, which is not good, hence the term catastrophic. The best way to know if you're bike is truly trailworthy is to take it to the LBS (local bike shop) and talk with the shop guru and be realistic with him as to the type of beating you're gonna dish out on yur bike and see if he feels the bike will hold for you're purposes. Many of the guys on this forum can direct u to a bike shop they trust and can probably give u the name of the honch that u should talk to. If a new bike is in yur cards, do the reading, talk with the bike guru's and come up with a price range and a number of bikes that u should test ride and figure out which one feels best to u and go with it.
I'm throwing a lot of crap at u, but really what it's all about is having FUN. If u enjoy the sport, then you'll do it more and get in better shape which is ultimately the goal. I highly recommend the beginner tuesday night ride at Allaire, very few of the riders can be considered true beginners anymore, but the pace is only as fast as the slowest rider, the breaks are plentiful and moreso if u need them, lots of good riders to get tips from and ride behind (as you've been told), and they/ we really just like to have fun and pass that fun along to new riders. There is a self torturing aspect to mt biking which kinda has to be appreciated to, but having testosterone in yur body probably makes us all accustomed to and appreciate that. Now stop reading this BS and go out and ride yur bike and have fun damnit!!!!!