Depending on your skill speed, this could easily be a problem. You'll definitely want to go back and get the LNC class up to get Invigorate... this is a must have for any TP-consuming class (save for tanks, since they don't have the cross-skill ability. Then again, it helps a lot on GLD and MRD when initially training them up).

With invigorate, and good timing on its usage, you can easily last 5 minutes before entirely running out of TP. Ideally, you'll burn invigorate by the time you're around 500-550 TP. By the time it comes back off cool down, you'll likely be around 200-300 TP. Use it as soon as it's available. The last burst will be even lower than that, so timing on it's use is critical. Again, use it as soon as it's available.