Paul's response is pretty good.
When I was training properly (and had the time) a lunch session could be 25min run to the beach 30 min swim then 25 min run return when the weather was nice. Still 50 or so mins run but it felt harder than a 50 min straight run and therefore of more benefit to fitness/adaption.
Personally, given your scenario I'd do the 45 min run (I'm assuming there are no shot cuts), swim as much as you want/need (Don't think it makes too much difference) but I'd run more than 1.35 home, perhaps 1.50 as a long run is a long run. My boundary is 1.40 for medium to long run.
With all that, and I'm sure you're across it is nutrition (or drink) is most important depending upon the temperature/humidity, you'll have to improvise well after getting out of the water. If not the run home could turn pretty nasty and my times above would not be recommended. If it is Melbourne cool it's not so bad - Sydney not so.
If your program allows it, a 45 min W/T aerobic session that doesn't impact your run (your fitness level will determine if that is possible) prior really piques off a good session.