There are certainly more qualified people than myself to speak to this, but I think it's a lot more complicated than just the calories themselves. Your metabolism will change as you go through it, and choosing foods that will keep it humming at a higher rate will pay dividends later on. If you imagine your burn rate as a graph, it isn't going to be linear, it'll be more exponential with results building on results over time. So while on a straightforward basis, calorie deficits will help you lose weight, it's not as simple as just saying, "take in x less calories per day." Your body adapts and will fight to preserve what it has if its threatened. I think that while calorie counts will always be important, you may find that it's not only the calories but where you get them from, and how you burn them that matters. For example, if you go out and beat the snot out of yourself for exercise again and again, you'll eventually fatigue and possibly injure yourself and if that happens, you've lost the exercise component of your calorie equation at least in the short term. But very few of us have hours and hours to do steady state cardio burns everyday, so you may need to find a way to balance hard core workouts with easier efforts and still find your way to the necessary burn.
If I were you, gvien the fact that the amount of weight loss you are targeting is pretty significant, I'd go talk to both a trainer and a dietician and be brutally honest about what you want to do, what you see as your biggest challenges in doing it (those can be anything -- from uncooperative or unsupportive family members to your own capacity to mess yourself up), and the reasons for doing it (e.g., "trying to become a better athlete" is a different thing altogether than "trying to become healthy enough to continue living".) The fact that you already ride means you have a certain athletic base that your body has already adapted to (otherwise, you'd likely be well on your way already based solely on your exercise.) So you may face additional challenges that people who weigh the same as you but aren't active don't need to worry about -- their bodies will respond quicker because what they would introduce -- activity -- is something you're already doing because you ride.
Again, there are certainly more qualified people than me on this stuff, and I'm sure a few of the folks on this board can probably point you in the direction of whatever you need to get started. But I do think talking to a professional will only help you get where you want to go in as safe and effective way as possible. Good luck with it and keep us posted on how it's going!