What is the recommended amount of exercise for seniors in a day?

It’s recommended to do 30 minutes of exercise 5 times a week. This doesn’t necessarily mean training at the gym or running, but just doing an activity that increases your heart rate and gets you moving, for example.

2–3 times a week is ideal for a beginner runner if you can space it out throughout the week (run one day; don’t run the next, etc.). It lets your body start recovering after each run. And those “rest” days can be active rest days where you walk or stretch to help your recovery. You should be recovered from the last workout (1–2 days); if not, it probably means you did too much if it’s taking you more than two days not to be sore.
It’s better to take the first two weeks to ramp your intensity up preventing you from getting hurt.