Gears and Cadence

I have been riding for a while and not really "schooled" in riding techniques etc. I focus mostly on building distance but I do watch the progression of my speed, I just don't focus too much on maximizing it. I am generally in the 15 +/- mph range. I also look at my cadence but never really worked to change it until recently. It has gone from the low/mid 50s to about 60. For the most part my only change has been to go up hills in lower gears/faster cadence. I have seen references that cadence should be in the 70s to 90s, some say even higher. - I say "holy smokes" that will be a big move for me. So now I am going to work on riding in lower gears with higher cadence (hope it doesn't make me nuts), I am sure that it will take a few weeks to get used to it. What should my expectations be for its impact on my speed and endurance? I expect that my endurance will go up and that I will be able to add miles and get over hills more confidently, which would be great. I don't know what to think about the impact on my speed. I would expect my speed to go down with the lower gears but I hear it should go up. I am sure that physical condition is a factor. I am in my mid 50's, 6' 0' and about 230#s. I ride a Cannondale CAAD 8 - 2 rings in the front and 8 cogs in the rear. Thanks for your comments.