When I went out yesterday, I ran through both of my 10 AH batteries, and got partway through my "emergency" 5 AH battery. Actually, even if I did totally run out of battery, I still have a working bicycle.
Today, I checked on Hobbyking, and they've got a special on the 10AH batteries that I like. I get 10AH, 14.4 volts for £22 (usually £43). Three of those gives me enough battery for a few hours of biking - of course, it depends on how much power I use. On a good surface (such as tarmac or gravel) that would be a lot less than on a bad surface (soggy mud or soft grass). So, I bought three more batteries to make aup another battery pack, and I also got three more chargers (I use chargers that cost only £6, and can charge up the 4S batteries that I use via the balance port, which makes it very easy for me to set up the recharge.