Matt
How many press ups can you do (to fail)?
Not sure mate as I don't do them any more.
I used to be able to do 150 or so and about 50 single arm ones.
I'd guess if I had to do them today I'd be able to do about 40 normal and zero single arm.
That makes me feel better
Do you think that with training I could be doing 100 by xmas?
Yeah you'll piss it.
Bodyweight excercises are all about practice and repetition. You don't need to be particularly strong overall just strong at what you're doing.
It's the same with chin ups. 6 weeks ago I couldn't do any so had to use the assisted machine. Now I can do 8 or 9.
Your body adapts to what you ask it to do on a regular basis.
Start by doing 10 push ups per night for 1 week. Tonight would be a great night to start. I know it's tempting to do more but just work the muscles and joints slowly and don't risk injury.
Add 5 to your nightly goal every week. Next week starting Monday do 15 every night. Again don't try to do more, you only risk injury.
You've got about 24 weeks to Christmas so you'll be well over 100 doing it this way.