R,
Congratulations on your PR!
There could be quite a big difference between your custom Level 2.5 and Level 3. If you feel you can meet the demands of the Level 3 plan (in the book) then that’s the best option. It’s pre-made, no need to make custom workouts or decisions, it’s already laid out for you.
If there is a chance you won’t be able to keep to the Level 3 plan, then lengthening your existing workouts is a great option. For many athletes, it’s much more convenient to increase the length of one workout a day than to add a second workout.
David