Recovery day:

Find a 30 minute yoga flow online and follow it.
We like Yoga with Adrienne on YT.  

Swim or Z1 Cardio:
30 minutes to an hour.
If you're trained hard the past two days, you'll probably need recovery today, so go gently
(Zone 1).

If you missed training yesterday and need a challenge, go Zone 2 from 5 mins to 3 hours.

If you are feeling punchy and want to mix it up, swim or run a Farrtlek session, 50m fast, 200m recovery x 8 or as many reps as you can fit in the time you have available.

Why recovery?
Training is designed to cause stress, recovery builds you back better, stronger, fitter.
No recovery = no improvement, just degradation, illness and injury.

For most of us over 40s, two days of training demands a recovery day. Some people might fair better on one day hard, one day easy. You have to adapt to the reality of your situation rather than following a program that doesn't take your recovery status into account.
We program active recovery which will speed up how fast you recover, but you could just take a day off. Keep eating the protein on your recovery day. Sleep as long as you can.

You can adjust the challenge any workout places on your body by adjusting the intensity right down and limiting the volume.

Newbies:
Sorry to break it to you, but you're not 21 any more and you're not an Olympian.
Train smart & build in recovery or smash face first into a solid wall of fatigue (somewhere around week 3 usually) then grind to a halt, get ill & probably injured. Those are your options.

Photo by Martin Sanchez on Unsplash