Bikes are a great way to up the cardiovascular fitness without getting injuries (unless you crash, cycling is either no-impact or incredibly high impact

).
That said, cycling is not going to help your running weaknesses at all and may actually contribute to more running injuries. In my case, I was a relatively serious cyclist for a few years (6000-7000km per year across road and MTB) before I started running. Got plenty of ITB issues when I started running because my cardio fitness was good but all the supporting muscles in the legs were weak because they are not used when you cycle because your movement is constrained. Had to run what felt like really slowly for ages just to build up the supporting muscles (by really slowly I'm talking like average 125-135 heart rate at most for up to an hour at a time). For reference in a 3 hour bike race my average heart rate will be around 155 - 165 so the 125-135 felt like I was taking it really easy and got fairly frustrating.
As far as the half iron goes, yeah, you need to put in the time in training or the race itself is really going to suck. Last year I did my first half iron and I did 166 hours of training between 1 Jan and 1 June for the race on 2 June and I actually really enjoyed it. Took it relatively easy and made it in just under 6 hours but I could definitely have pushed quite a lot harder. That said I was carrying quite a lot of cycling fitness when I started training but I basically started running and swimming from scratch (ran 10km between May 2018 and Dec 2018 and swam 3km in the same time period).