Stretching is a very valuable and extremely beneficial ritual I use before every and any workout. No matter what it is you want to be relaxed and not tense for any exercise. Try it and see the difference.
Before Tai Chi you could practice a ritual like a couple of minutes of mindfulness, meditation or controlled breathing. Before the walk you could stretch your legs a bit ot do a couple of jumps.
I usually meditate and do yoga first before exercising because it indirectly leads me to present moment and also my body will be fresh and ready.
You could a breathing excercise before the tai chi and walking. Also streches afterwards are important as well to prevent muslcle ache