I had never done any biking when I did my first tri. Although your bike leg is much longer than mine was. I would consider looking into buying a good quality road bike, and then just get out and ride on it as much as you can, to build a good base. Once you get to the point where you feel like you have a good base you can do some intensity training. For example if you are doing rides 4x week averaging 1.5-2hrs, you could switch one of those longer rides with a 1hr workout where you break it into intervals. Ie. 10min warmup, 5x5min at 5-6 RPE with 3-5min rest, 10 min cool down. Something like that.

Once you get comfortable with doing 1 intensity workout you can increase to two per week, but I would vary the two intensity sessions. Do at least 1 outdoor bike ride a week. If you have no cycling experience than you have to consider yourself a beginner and keep your progress realistic. If you enjoy the sport, you might find yourself getting much more serious about it. Good luck, hope that helps.