Hi Ben
The bike is a major part of a triathlon, especially if you are already a competent swimmer.
1) you need to be good on the bike to be good at the bike
2) you need to be good on the bike to have enough left in the tank for the run.
Of the 3 disciplines cycling is also my weekest ... so i spend almost half my time cycling in some sort of way.
When yoy switch to Tri training you no long need to rely on running for overall fittness improvment as this will be gained from cycling ... so most people comming from a running background can drop back on the frequency of the runs.
A popular pattern is 3 of each per week .. then maybe an extra of your weekest. .. occationally cutting back on your strongest if needed.
Also it can be benifitial to move 1 of your runs to after a bike ride (a brick sesion) see point 2
have fun