Diagnose why my run sucked

Today on my long run for the week (7 miles), I stopped at 6.25 miles into it to deal with a cramp in my diaphragm that had suddenly intensified. I had been cramping somewhat in various locations (stomach, diaphragm) since about mile 1 but was able to keep them under control by keeping my pace slow and breathing deeply. When I stopped, my legs felt like they were going to give out under me and it took ridiculous effort just to start walking again. Felt like I had 20 lb weights on each one of my quads. I've never had this sensation before so I figured I would turn to you lovely people for opinions on what happened.

I'm a pretty slow runner so my average pace for today's run was 11:15 so I whimped out of the run about 70 minutes into the run. I didn't think I needed to worry about calories on the run until 90 minute runs or so. I've been eating/drinking my normal amounts of food/water the past few days. Today was a little more humid than I would like but nothing that is unusual for this area. Should I start taking in calories early possibly? Eat more? Just suck it up and get better at running?

Background: I started running August 2010 working my way from a Couch to 5k program to a few 10ks last spring. I slacked on running milage during the summer as I picked up swimming and biking workouts. The past few weeks I've been back to focusing most of my time/effort on running. 

Running distances the past few weeks: 1, 4, 1, 2, 1, 3 (total of 12); 2, 5, 1, 3, 3 (total of 14); 6, 4, 3, 3 (total of 16). This week I was going for 3, 7, 5, 3 for a total of 18 miles. On my long run last week of 6 miles I felt great.