Fitness points and age are only calculated for running and cycling activities with an HR above Zone 3. Otherwise it will have no impact. Even when it does, it seems to be a random, capricious measure that ties to very little. It is based on whatever algorithm TT came up with, which is different than what Garmin uses or Strava uses, or anyone else. Ultimately I don't expect a watch to be able to tell me how fit I am. My suggestion is to ignore it as it is meaningless.
I imagine this is not what you were hoping to hear but let me know if this answered your question. If it did, please mark it as a solution so other can look for it if they have the same question.
Thankyou! I thought as much. Feeling a hell of a lot fitter and that's what matters. Just a pity tomtom could not get this correct.