I'm thinking it would have been substantially faster than his 3:43.13 WR. In his 1500m WR, he averaged just under 13.8 seconds per 100m. He would have 109.34m to go.
You could look at it two ways:
1. The actual way it would happen: he paces himself *slightly* slower and runs the mile in x time, hitting 1500m at a slightly slower time (which is obviously how he ran his mile WR - a 3:28.21), or - (and the point of this thread):
2. Let's say he runs 3:26.00 and keeps going - which I know he would never do, but the point I'm making with this scenario is that if he did do it exactly that way, I believe he still would have the gas to have broke his mile WR by a fair margin, as all he had to do was run 109.34m in 17.12 seconds to break it. I've watched the race, we all have, and it's not like he crosses the line and falls over completely gassed. He runs over and grabs the bouquet of flowers and gets congratulatory hugs, is smiling, seems quite in control.
17.12 seconds to run another 109.34m when he had averaged under 13.8 seconds for each 100m of the race. I say he has a good chance at 3:42 flat, and maybe even 3:41.9x.