First "real" training workout
Here are my thoughts, I'm sure plenty of of other people will have different things to say.
1. If you're just starting biking, it is perfectly normal for your legs to get tired. This is why we train. It takes time for you to able to increase your speed, strength, cadence e.t.c. The key is to understand what each bike ride you are going is doing for your body. It is much better to have structured work-outs then to simply 'go at 'er'. You need to mix in long, slower recovery rides to build your aerobic base, have shorter, but much more intense workouts, and mix in hill workouts too. There are plenty of training tips for biking in the training section of this site. Read them. Secondly you are on a mountain bike, which although you say is light, is still not comparable to a road bike. Its fine to use, just don't expect to be breaking any records on it.
2. Basketball training isn't going to be that bad. While not as good as simply focusing on triathlon, it is a cardiovascualr sport and will benefit you. If you can I would still try and fit some swimming, biking and long distance running into your bastketball training, even if it is just once a week for each.
3. I've heard this too. I am 21, and I have done a few sprints and Olympics, and am doing my first half-IM on Sunday. I haven't found it to be a problem yet, and I think that as long as you train carefully, and don't try and do the longer distances until you are definately ready, then you should be ok.
Good luck.
Brian

I really would like to be participating in a few Tri's by the next season in 2006. I have decided that today would be the day to start biking since I had already been running about 3 miles a day. I rode roughly 15 miles today with a stop for a 2 mile run at around 3/5 of the bike distance. I just have a few questions for you triathletes who have been training for this for a long time.
1. I noticed that my heart rate wasn't the problem with my bike speed, but my leg strength was struggling to keep the pedals going. Is this the case with biking period?? I was pedaling on a Gary Fisher mountain bike that is fairly light and doesn't have any springs or any sort of heavy things to weigh it down. I plan on getting a bike better suited for longer distances eventually but I just am wondering if it is normal for biking to be more leg intensive than heart intensive, or if it is just me not having any experience on the bike??
2. Will the training I am doing now be all for naught when I get into basketball shape? I start team training around end of september and then the season will start shortly after. I am just wondering if my basketball mentality and conditioning routines will "kill" the training with a bike now?
3. I am going to be 19 soon and read that younger people should not really participate in longer events for sake of injury. What kind of Tri's do you suggest that I start out with since I am a very active person and am pretty slim for my height at 6'6 and 200 lbs? I eventually want to get into some 1/2 IM's and maybe later on down the road in a few years do some full IM's.
Any feedback would be greatly appreciated. Thank you