How hard should it feel?

Each plan has its own effort. Here is how to tell, starting with the talk test.

In tai chi walking and chair yoga: a gentle effort, never a strain. The talk test: you can talk in full sentences but not sing. That is about right. If you can't talk, slow down.
In calisthenics, the talk test: you can talk in short sentences. If you can't, slow down or take the rest. Form first: stop a set when your form breaks, even with time left on the set.
Breathe all the time. Breathe out on the effort. Never hold your breath, above all in planks and holds.
Move in a range that feels comfortable.
After seated moves, stand up slowly. Before you sit, step back until you feel the chair on the back of your legs.
Balance moves get harder by time, never by letting go. Keep your hands on your support.
Drink water, even if you don't feel thirsty. If your doctor limits your fluids, follow their advice.