My Pro Coach training plans range from 8 to 40 weeks in duration, so the best thing to do is count back from your "A" target race date and then choose a plan to fill that gap. You can fit your other races into this timeframe.
The training plans take you right up to your race day (aside from the Winter Base Training plans). The longer the plan the easier it starts and the more gradually it progresses. So that has a bearing on the difficulty level you choose. For example a 12 week plan starts off at a higher level than a 36 week plan because with 12 weeks left there's not as much opportunity to build up slowly.
What about when you have two "A" target races in a season? For example an Ironman 70.3 and an Ironman 140.6. Which training plan should you pick?
In this scenario, if the two races are within 10 weeks of each other, you should stick to one Ironman 140.6 plan (the more weeks the better) and this will also get you in great shape for your Ironman 70.3.
If there is a gap of several months between the two races, you could choose two separate plans - an Ironman 70.3 plan and an Ironman 140.6 plan.
When it comes to choosing your training plan, you should always pick a plan for the longest race of your season. In the above scenario this was an Ironman 140.6. And the same rule applies for Sprint and Olympic distance triathlon training. This is because once you're fit enough for an Ironman 140.6, you're certainly fit enough for an Ironman 70.3, but the same can't be said if that scenario was reversed.
All the training plans include speed work, so an Ironman 140.6 plan (for instance) will still get you in great shape for an Ironman 70.3 and even an Olympic distance race.