How much is too much?

Pain, that is.

Today I ran my first day of longer intervals (400m). In the past I would do 1minute intervals and never had a problem. That second minute is brutal. So here's what I don't get: my average speed for my workout was slower than my Sunday run (given the slow recovery periods) but by the end of the workout I was totally destroyed. I could not have run another round, and when I tried to just do one or two quick 200m intervals, they were a no-go as well. At first I took this to be a sign of success. The one mile cool down was tough, and I basically just slapped my feet against the treadmill belt until I could justify stopping. Normally, when I take off my shoes my feet instantaneously feel better. Not so today. In fact, my normally well behaved foot has gotten steadily more painful as the day has gone on. What am I doing wrong, people? Someone please tell me... Is it possible that the treadmill isn't good for speed work? This seems counterintuitive to me. But I can't afford more pain like this. Pain in my joints (ankles, feet, wrists, and hands) really gets under my skin; it makes me think about my parents' arthritis. And I plead silently to the powers that be: Not me! Not yet!

Is this why people take ice baths?