I just did my first sub 20/5km - 19.47, so 3.57 pace. But could barely manage that pace for 400m intervals and could only manage 4min/km for 1km intervals. Obviously had me concerned - how can I run 5km all under 4min/pace when I can't even manage 1km at that pace with a rest....

Not sure why that is, I'm guessing a lot of variables to consider. Perhaps I just need to work harder in training, but I try solo and would most likely do better if training with others. I do race quicker, found it easy to sit behind someone at 3.55 pace, couldn't do that in training.