I'm in the same boat my friend. I spent about $150 on a dept. store mtb, helmet and a couple tools. I rode like 3 miles for the first few rides (over a week), then just 'went for it' and did like 15. I felt great for like 3 days afterwards. If I can pass on some advice: stretch before and after, or you'll pay for it the next day. Take as much water as you can, you'll need it. Travel light, but be prepared: flats, slipped chains, etc etc ruin a ride quick.
Diet is the biggest part: drink lots and lots and lots of water, cut the pop and alcohol, those are the calorie killers. healthy snacks between meals are key for me: grapes, baby carrots, fruit and nuts are slowly taking the place of the junk food I adored for so long.
keep at it, set your goals REASONABLY. There was a guy in Canada named Terry Fox, he ran across the country to raise money for cancer. People asked him how he did it, his answer was 'one telephone pole at a time'. it was meant to signify that small steps are the key. by the way, he did it with one leg, ran like 3000 miles before he had to stop. What we're up against is really nothing.