He means 'a mile in 6.12' not 6.12mph. It's about 11mph (12mph is 60/5 and so a mile in 5 minutes, mile and a half in 7.5). But what you want to do is get to a running track and think 'it's 2400 in 9:18 (55
so 400 in 93 (1 min 33) or 100 in 23 and a quarter. Bring a stopwatch and work to those times. How I did it was get the 400 time on its own first, then do six of them with rests in between, then cut down the rest time until you're doing the whole thing at the needed speed.