Retired AF guy here (although I was current when I retired). My advice:
1. Get all your ducks in a row if you haven't already. Make sure you have a fully-polished resume (both flying and non-flying versions). Start ordering the other stuff you'll need if you haven't already (copies of college transcripts, driving record, FAA records, etc). You won't be handing over your driving records, FAA records, etc, but if there is something inaccurate in there you need to know about it and get it corrected.
2. Apply as early as possible, although most Majors won't even consider you. When you go online to apply, most have a list of 5-10 questions that you must answer yes or no before you can get to the application. One of those is often "Do you have at least 100 hours in the past 12 months or 200 hours in the past 24 months?" If you answer "no" to any of the questions, you cannot even fill out an application.
3. Don't forget about corporate aviation. If you have lost touch with your old military buds, get back in touch pronto! Networking is EVERYTHING in the corporate world! You might be able to find a corporate gig where they will pay you peanuts to fly in their right seat -- just make sure the aircraft REQUIRES a right-seater so you can log the time. That would allow you to get the needed recency -- it is probably going to be way too expensive to buy the recency you need. Also, keep in mind that corporate operators HATE guys who use them to get their recency and then bolt for the airlines, so your best bet is probably to be honest with them about your goals and offer to give them a reasonable amount of time (18-24 months, maybe) before you decide what you want to do. You might love it, so keep your mind open.
Important question: How long before you retire/separate? If it is less than a year, then you are WAY behind schedule!
Finally, as you are filling out your first application, I recommend that you create a "master application file". It was very helpful for me -- basically, I put all the data I gathered for my first application into a single word document (addresses the last 10 years, info about references, etc, etc) and then continued to expand the document as I filled out more applications with more data requirements. This makes it WAY easier to fill out subsequent applications so you won't have to dig up the same info over and over.
Good luck!