Weird. I had no idea they had Target stores outside of the US. That kinda sickens me(though not as much as Wal-Mart stores). In any case, I say take the bike back if you just bought it. Don't feel bad At All about taking back a lightly used bike to one of these big "box store", greedy, earth destroying, sack of s**t companies. Then save yourself a little more money and buy a better bike from a local bike shop. You'll have a Much more reliable bike that's actually built or trail riding. But even if you don't do that, as DX-MAN said, it's just called mountain biking. I personally wouldn't really call it an "extreme sport" unless you're doing some pretty hardcore downhill or something. It's just having some awesome off-road fun on a bike with knobby tires. As for tire size, you'll probably do much better on at least 26" wheels. That's what I'd recommend. 29" is also very popular, and definitely has it's place. If you're racing or get a thrill out of going as fast as possible on smoother XC single-track, that's probably your best option. If you just want to get out and have fun on some dirt trails, whatever type they may be, I'd say go for 26". No matter what your choice, good luck and have fun. If you keep the bike you've got, still ride the hell out of it and don't listen to anyone if they make comments about you riding a "low-end" bike. If you're out having a good time and the bike works for you, that's all that really matters. Seriously. Just have fun.