12 miles on a bicycle

I am a very goal orientated person. I have discovered this about myself the more I run and try other activities. I am still unsure how fast I should be aiming for on my bicycle. I can tell you one thing, you use different muscles when you bike compared to running. Today I set a time goal of 60 minutes to finish 12 miles. I beat  my goal at 55 minutes and 39 seconds which averages out to 4.6 minute miles. I average about 13 miles per hour. I have to admit, when I got off of my bike, my legs were shaking. I cant imagine running a 5 k after that but I have until July 2nd to get my speed and the transition for running and biking squared away.