I get 39% of my High Three average civilian pay for life though indexing the benefit doesn't start until I'm 62. There's also an option, with a reduction in annuity, that is like military SBP; lets my spouse get 50% of my civil service annuity if I die first. I bought back ~20.5 yrs for civil service. Your Minimum Retirement Age (MRA), 57 sounds about right for you, is when you can get an unreduced retirement. Check the OPM site; you can retire earlier than MRA if you meet specific age/years standards, but with a reduction or delay in annuity. I retired at 58.5 years oid. That means I get my civilian annuity now, but not my military annuity until the beginning of 2014. For health care, I keep my FEHB Blue Cross coverage until I'm a retired military guy when I'll suspend (not cancel) my FEHB coverage and go to Tricare. I don't know much about your situation...location, job, support for your military participation from your civilian leadership, your career desires/plans...but one approach would be to fence some of your severance to buyback your AD time, work until at least MRA (57) and continue your military career, unit or IMA, at least long enough to qualify for a military retirement. As time goes on, other doors may open and you can adjust your plan. As mentioned by Nords, TSP is an outstanding benefit and contributing enough to at least get the 5% matching should be a priority. You also have the new Roth TSP option to ponder. Lastly, if you qualify for a military retirement, I believe DFAS will recoup the pretax amount of your severance pay from your military annuity (max rate of 40% of the monthly amount) once you start receiving that check. Check this link for the DFAS guidance:
http://comptroller.defense.gov/fmr/07b/07b_04.pdf.
Lots of things to consider as you look at choices, but, since you currently have a GS-13 position (I assume a permanent appointment), you have a good platform from which to formulate your plan.