Panic not, you can always add and travel extra trains to your pass. The trip is simply the overarching location for your plans. Nothing in your trip is valid until you add it to a travel day in your pass, and is a bit like a diary for sending birthday cards - until you post them you can change at will or even send to somebody not in the diary.
You add or remove journeys to your trip (Like adding new birthdays to your diary), which can be single or multiple train rides and either pre planned or taken as you need in real time.
Read the guides on activating and adding train journeys from your “trip into the activated pass, especially the advice not to activate your pass early or add individual trains to an active day until you know you you will be travelling on the train.
Trains can be removed or changed in your pass if you change your mind,.
Note if you have a flexi pass and a travel day has been activated, and you then decide not to travel that day ,you can only recover it if you cancel before midnight of the day before. You lose the day from your allowance.