Yes the LSR is the most important thing but most people wouldn't advocate it being such a high percentage of your overall week's training. Even without speedwork, more easy miles would be beneficial.
I think it's a fair assumption that you wouldn't have got cramp if you had managed to get more miles in overall and improved your endurance. If you manage 50miles/wk this year, I expect you will have a much better run.
I peaked at 58 miles/wk last year and felt great running along the prom and picked my pace up for the last three miles.
Dr Dan - yes I did feel bad. It started during the recovery after our first of three mile reps and continued into the start of the second. I felt a bit lightheaded and nauseous and I did feel like I might collapse. I was very glad not to be running on my own and that Curly was just in front of me. However, a couple of minutes later I felt ok again and I completed the session with no further problems.
I tried to tell myself I'd imagined it and it was only when I uploaded the Garmin data and saw the high HR trace that I realised that something may actually have been wrong after all. In all honesty, I was quite frightened by how I felt at the time. As Curly said, I'd not noticed anything like this before.
I've been looking at the AF thread on Fetch and your pSVT thread on here and do think it sounds similar to what you experienced. I think I need to get registered with a GP...