In my humble opinion:
Diet is where you can make bigger differences over exercise. However, genetics can make a bigger difference than both diet and exercise.
Diet:
Cut back on processed foods and limit your consumption of meats (having fish, turkey, or chicken whenever possible). Incorporate more leafy greens as often as possible. Drink water, seltzer, or unsweetened tea (with lemon is great) whenever possible. Eat lower sugar fruits whenever possible (blackberries, raspberries, strawberries, kiwis, avocado, grapefruit, and watermelon). Keep your fiber intake up.
Whenever possible limit the amount of hours you eat or drink calories (in example: 10 hours of eating / drinking calories followed by 14 hours of fasting. Even better is: 8 hours of eating / drinking followed by 16 hours of fasting. If possible get down to 6 hours / 18 hours a day or 2 a week.
Exercise:
You'll need to increase your heart rate in order to really get the most benefit from exercise. If your Fitbit doesn't track your heart rate, get something that does. If you are a man, the formula for your maximum heart rate is: 220 - your age. So if you are 20, your maximum heart rate would be 200. 30 years old = 190. Forty years old, your maximum is 180 bears per minutes, etc... For a woman it is a more complicated formula, look it up online. I've attached a chart from my Garmin app that explains what each zone of your heart rate affects your cardiovascular fitness (see attached). If you are below zone 3, you aren't making any improvements. Start off slow and build up to higher zones.
Genetics:
My Girlfriend had done an Ironman a year for 9 years in a row (her mid 40's into her mid 50's). Even during that heavy training schedule, with a clean diet, her cardiologist said she should be on a statin. You can't always outmaneuver genetics. Luckily statins are a good way to control cholesterol. They are tried and true, and even available as a generic. If needed, there is no harm in taking one as prescribed by your doctor.
Lastly, the ratio of HDL to LDL (good to bad cholesterol) is know as triglycerides. If your LDL gets to a good range, but you triglycerides are still out of whack, eat more plant based fats (avocados, nuts, olives, etc) in order to improve your truckers.
Please keep writing back in this thread as things progress and let us know how you are doing!
Good luck and I hope that I was helpful, Ken