I also have high blood pressure, high cholesterol and I am prediabetic (aka borderline). This is what gave me the motivation to start my weight loss journey. My blood pressure at the moment is controlled by meds but I hope to one day go without them. As for my cholesterol, my doc says to change my eating habits and it will decrease eventually. She is not too concerned about it. As for the prediabetes, if I lose weight I will be able to prolong the onset of diabetes. She doesn't think I will be entirely able to avoid as it I have a strong family history. If I can make it hold off until I am 80 that would be ok (I'm only 33 now).
As for exercise, when I first started, my heart rate would be up into the high 170s. Now it seems to stay around 140-150. Once it jumped to 210 and that freaked me out a bit. I don't want to have a heart attack on the treadmill (people with prediabetes are 3x more likely to get cardiovascular disease i.e., heart attack or stroke).
I am starting to feel better now, you will too. Make sure you bring up all of your concerns with your doctor.