Starting from sedentary

Hi everyone, I recently decided to start improving my fitness due to health anxiety.

Up till now, my only exercise has been 2 45-min to 1-hour walks daily, once after lunch and once in the evening (I only have breakfast and lunch, no dinner). The rest of the time I'm literally at the PC; sometimes I remember to get up and stretch and move a bit, sometimes I don't.

I started trying 10 push-ups, sit-ups and squats 3 times daily, and found that my chest (pectoral?) muscles and some arm muscles were burning by the 2nd set of 10 push-ups on the first day and I couldn't do the 3rd. The squats and sit-ups were no problem though.

How normal is this, and are there any changes you'd recommend? I'm looking for exercises a total beginner can do at home without any equipment, focusing just on improving health and fitness.