Living off of rental properties for most of retirement income?

[Steve C.]

In my field (counseling), I've been working for almost 20 years.  While the salaries have been low, up until this year when things really spiked, the retirement plans and money able to be set aside was just embarrassing!  It's to the point where, had things not changed for me as far as my income/earnings...I'd have had to pick a new field or else by the time I retired I'd have enough saved up to support me for a year or two at most.  

As I said, now I'm in a very different position financially and I'm saving up to buy my first rental property.  It'll actually be an investment property that will serve two purposes.  First floor will be used for my practice for office space.  Second and possibly third floors will be for 2-3 apartments/units that can be rented.  

At this point my plan is to let a portion of the tenant's rent pay off the mortgage.  But thinking ahead...if my plan works and I develop the ability to buy multiple properties....lets say having a total of 15-20 apartments spread throughout 5 or 6 buildings in my area... maybe that could produce a very comfortable continuous income for retirement??  

I figure that in 20-30 years all the mortgages should be paid off and whatever I get in rental payments, aside from the big chunk of repair/insurance/legal costs...should be 4-5 figures each month.  

Does this sound realistic and can anyone speak from experience as to how this works...or doesn't work?  What would help me to learn, work towards, etc. that could make it work?