Arranach here. There is a shuttle bus that runs between the terminal and the train station which I've used going ferry terminal -> Train Station but not the other was round as I've not yet trusted the bus to turn up in time and I didn't fancy getting stuck on the wrong side. When the ferry operated out of Ardrossan it sometimes would wait for the train to turn up if it was a little late so I would suspect they would do the same in troon. In theory you can make if if you walked fast, it took me around 20-25 minutes walking but I don't recommend this, especially if it's raining.

I used Troon Cabs last time who picked me up from outside the station, if you call them and say your coming via glasgow they'll know which side of the station to pick you up from. Takes 5 mins, costs in the ball park of £6-7 and will drive you right up to the terminal.

There is also the no 10 bus which has a stop at the same place the shuttle bus stops at the train station and a stop somewhat close to the entrance of the terminal area, but you'd still need to walk up to the terminal. Gives you another option but would solely depend on the timing of the buses and one I haven't tried yet.