I use my Garmin c330 gps to track what I do(I bought it for Over the road trucking) I just put it in my side pouch on my bike and take off. That is why I look at miles, but when I do ride I go for at least a hour, and when that happens to be in town, I only get about 12 miles. This last trip was out of town, so I could actually get 15mph average, even up and down the hills.
This particular 20 miles I have been eyeballing(and wanting to do) since I was about 12 years old. Then I stopped riding at 14 when I got my car, then I started riding again at 33. So one could say its been a goal of mine for many years.
What I am starting to find amazing, is I will do 12 miles, and I look at the time, and its LESS than an hour! So I decide to ride another 3 or 4 down to the store to buy some burritos or some such thing I need, and then I ride back.
And about slowing down. I just cant do it. I absolutely LOVE pushing myself as hard as I can on my bike. I get a side pain at least once a ride, then I remember to control my breathing(and that is not very easy sometimes) but I love it. I kind of hate running, but being on my bike and pushing it up to 25+mph on the flat(for me, thats fast)...shivers down the spine. I look back and don't know why I ever stopped doing it.
I have been thinking about it for a while, and I will take your advice paednoch, my next goal is a 2 hour hard push. The only problem is I don't have bike shoes yet, nor clipless pedals, so I have to stop for a minute or so ever 5 to 7 miles to un-numb my feet. I'm not sure if thats from riding so hard for 6 miles, or from my recumbent, but I've been wanting bike shoes since about 2 weeks after I got my bike.