I'm impressed you could run with yours in your hand! I do that sometimes for a little while, but then I get to sweaty and slippery.
I tried an armband, too, but I couldn't make it tight enough. Now I use a SPIbelt, which I love.
I have a rubber/silicon case for it that makes it easier to grip.
I also have a feeling I will have no trouble making the armband tight enough.

I have a SPIbelt, that I used for the Princess, but it won't fit this phone with the case on. Plus for long runs, since it is NC in the summer, my training group requires us to either wear a hydration belt or carry water. I use a one botle fuel belt (that I got on ebay for $9!), and the pouch is needed for gu and keys and insurance cards. Wearing both makes me feel too much like a pack mule!
For the Princess, a hydration belt is unnecessary unless you really need a LOT of water. There is plenty of water and powerade along the course for my needs anyhow, plus in Feb it will likely not be very hot. Disney is excellent at running races, so you will never have to worry about them running out of water, cups, etc. like I have read about at some other races.