1. Definitely not too old - I went into the Marines at 24 and ended up staying for 10 years. In some ways it helps as your brain is presumably a bit more fully formed and consequently you should be a little less vulnerable to the mind games.
2. Check some of Al C's stuff. But I think s&s plus some pull-ups, chins, runs and rucks gets you 90% there. Don't overdo it and show up burnt out - that's key. Not sure where you're headed but at the first few weeks are a stress test and you don't want to show up mentally or physically tired. IMO opinion better to be slightly undertrained than overcooked.
3. I think most important - you have to be 100% sure you want this. Uncertainty tends to be lethal. Talk to some vets and be sure you're doing it for the right reasons. It's a pretty crazy commitment and training staff will be right up in your grill looking for even the slightest sign of doubt or weakness.
3b. If you are a fan of Marcus I think you'll find the stoic philosophy quite helpful. Some sort of mental anchor is important as your emotions are going to be all over the place during boot camp / ocs. If your at least quasi-fit and mentally centered you'll be well on your way.
Best of luck to you with whatever you decide.