I am actually finishing up an article on methods of extending your Training Plan, but here is an excerpt relevant to your current situation:
If your goal is to build further fitness leading up to your “A” events, then adding additional Build Phase is the best way to do this.
It should be noted that the Build Phases are the most physically demanding of all TrainerRoad phases, so adding additional Build should be done with caution. Spending more time in the Build Phase than your body is able to effectively recover from can lead to fatigue and possible overtraining. As always, be sure to listen carefully to the messages your body is sending, and take extra time to recover when needed.
(1-2 Weeks) Repeat Single Weeks
If you only want to extend your Build Phase by a few weeks, then repeating single weeks in the Build Phase is a great way to do that. The Build Phase, by default, consists of two training “blocks”. Each one of these blocks contains three work weeks followed by a rest week. When we are looking to extend the Build Phase, this ratio will become 4 work weeks to one rest week.
The way we will do this is by repeating the week immediately preceding the rest week. In other words, you will repeat Week 3 or Week 7 depending on which half of Build you are looking to extend. This will continue to increase your 6 week avg. TSS before dropping you into your rest week. When adding a week in this manner, your weekly schedule will be:
Week 1 -> Week 2 -> Week 3 -> Week 3 -> Week 4 (Recovery)
You may do this adjustment during either, or both, halves of Base depending on how much time you need to extend your plan by.
It should be noted that increasing your work to rest ratio will increase the difficulty of your training by increasing your work to recovery ratio, and you should only consider this option if you have been successfully completing your planned training up to that point.
I hope this helps!