Speed drills = death

Well, now that my marathon training is over, I've decided to speed train. I am hoping to do my 5K faster. I did my first speed drill at the track last night. I seriously thought I would die. I did this thing I read in one of my magazines. You run around the track once as fast as you can and then jog 1/2 way around the track, then start again. I could only do 4 reps!! I just dont understand how I can run for 5 hours straight, but I cant go fast for very long. I may ditch this speed thing and try for an ultra marathon. 50 miles Baby!! I guess I need to get new tennis shoes too, since I figured it up and I've put almost 400 miles on the ones I have.

How far can you run without passing out?