I've always done a shorter faster run on Saturday before my long run on Sunday. I was talking to a family friend at Christmas (who is a pretty decent runner, has run 28 something for 10k) and he swears by doing long runs on Saturday and a pace run on Sunday when his legs are tired - says it simulates racing. He does this every week.
Anyone else do this? I worry it would just lead to injury by basically having back to back hard runs.