if you're 5'11'' and 150 pounds, you can't possibly have too much of a gut. just keep riding, eat well, and do some core work if you like.

i think the right cadence is whatever feels comfortable, so go with what works for you. 80-90 rpm certainly isn't "grinding" up hills, but there's nothing wrong with gearing down and spinning faster if you want. if you feel like you want more power and want to do some strength training, then squats (WITH GOOD FORM) and lunges will help, but they aren't necessary. just riding will get you there, too.

if your max HR is about 205, then 180-185 seems a little high for training rides. that corresponds to about 88-90% of your max. for comparison, i usually average around 75-80% of my max for base training rides. i don't know how hard you're pushing yourself and you're really the only one who can determine your workout intensity, but make sure some of your rides are at a more relaxed pace to allow your body to recover.