The trick with taijiquan is to keep your training mild.
It is not a gym workout.
You are not meant to be sweating and straining.
Go easy on yourself.
Little and often is the key.
If the training is gentle but works your muscles nicely, then it is easy to sustain and your energy levels will stay high.
Overdo-it and you will suffer from fatigue, aching joints and sore muscles.