long ride Energy, HR, Fueling & Cramping your analysis please

Folks,

I did my first long ride (100 miles) this season (second century ever) with mixed results. This is my second full season of riding so I have limited experience at this. I am contemplating a Ironman race in 2010 and want /need to start testing myself. My goal for this ride was to keep my HR down and let the time / MPH fall where it falls. 

At 5:00 AM I ate a banana with peanut butter and some gatorade. At 5:30 I took 2 Endurolyte capsules and headed out on the road. I had 2 bottles of gatorade and 2 bottles of plainwater that I alternate sipping every 10 minutes. I took an enduralyte capsule every hour I ate a power bar at the halfway point and a banana at 4 hours in. I finished in 5h:21m no stops and averaged 18.4 MPH, my average HR was 127. My right calf was a little tight but did not cramp I massaged it a couple of times on the road. I did get a viscious cramp in my left hamsting at about 6:30 Saturday evening.

I did not feel terrible when I finished riding, but I didn't feel like running a marathon either. The way my legs were feeling I know for sure that I would have cramped up if I tried to run. I am certain this is partly a fitness issue, but how do you push forward? Do you start adding bricks to the end of the long rides? How often should you do long rides? Is HR the best way to pace? What cadence? I wanted to stay between 80 and 90 RPM, but as I got tired I was mashing down in the 70's. When training do you peddle down the hills or coast. At about 80 miles in I started taking every opportunity I could to rest on the bike. I felt like the next time I should do this from the start. I was happy with 18.4 MPH should I give up speed for more energy conservation? Do you pace with ave speed? Thanks in advance for your opinions and help. I am 6'1" and weigh 248 pounds if this makes a difference for the fueling related questions.

chevy57