Generally people try and fit in a Half distance race between 8 and 4 weeks before the full distance. However there are plenty who's first tri is the IM.
Do you have any experience of shorter tris? Transition practice can be done at home as can checking out nutrition and pacing strategies but sometime only a race can show you the holes in these.
9 months is plenty of time to get the training on track, as others have said get the bike volume up as soon as you can and also if you need to sort out swim techniques the sooner the better before bad habits are ingrained.