On day one, do one push up. Day two, two. Sounds a bit ridiculous, but it gradually builds difficulty.
Crucially, it is not all in one sitting. On day 10, if needed, do five when you wake up, and five before bed.
Break it up into something achievable. And if you miss a day, don’t sweat it. Again, the idea is to start to build, or rebuild strength and flexibility, the exercises themselves barely matter; you could do planks instead, for example.
Don’t worry about what you can or can’t do four weeks from now. Stay focused on what you need to do this week.