I would treat your mortgage and you investments as two different things. All these new strategies/thoughts around bonds like counting debt as negative bonds, or counting SS as part of your bond portfolio miss out on the real purpose of bonds.
When stocks go down, quality bonds tend to go up. They are a hedge, and they are a hedge that pays you regular income. A hedge that pays you to hedge is a beautiful thing.
Stocks normally earn a lot, but they can be very volatile. Adding bonds to a portfolio of stocks decreases the volatility while giving up some of the return, but not all of it. How much you need to tame your portfolio depends on your goals and your tolerance for risk. Your mortgage, SS, age, etc. shouldn't matter when figuring out your AA. They matter for other things of course. And just to clarify, your current age doesn't matter, but your time horizon and how long you expect to need the funds does. Since this is the MMM community I don't need to explain that a 30 year old might plan to retire in 5 years. ;)
If you have a long time frame and high tolerance for risk go 100% stocks. Have fun. If you can't stomach a 50% drop add bonds to tame the portfolio. If you need the money in 5 years add bonds to tame the portfolio. It really is that simple....
The mortgage really comes down to your interest rate VS desire to be debt free. If your interest rate is less than what you expect to reasonably earn in safer investments then you should invest. If it is more pay down the debt. My general rule of thumb. Debt interest rate <4%=invest. 4-5% it depends on whether you would rather be debt free or have a bigger portfolio. 6% or more = pay down the debt.