How quickly should I pay down rental properties?

[Steve S.]

I'm evaluating how much longer I want to stay working and I have a portfolio of several rentals which will, in part, fund my retirement.  

If I continued to make the same amount at my job and rents stay the same, I would be able to put about $10,000 per month towards the outstanding balance of several mortgages.  

I owe about $650,000 in mortgages.  If i were to accumulate the money to pay one house off and then further increase my monthly savings until I paid down the next, etc, I could have all paid off in < 5 years.

Once paid off, I would gross about $105,000 per year.  2 of the houses are < 10 years old and 2 are > 25 years old.  All have new roofs, but regardless, for the sake of this discussion, let's assume the 50% rule.  that would generate $52,500 per year which is well more than I spend in a year and wouldn't require me to touch any of the stock investments that I have.

Interest rates are between 3.75% and 4.25% on all properties.  

What are some things I should consider before I ever choose to do this?  For now, I'm just building up cash as quickly as I can and this analysis assumes I don't add any other properties to the portfolio.  It also assumes I don't seel them.  I've owned all these properties < 3 years and have them on 30 year notes to allow me maximum flexibility should i want to pay down more slowly or add extra capital to them to pay them off more quickly.