Bicycle for 56 year old male

Question
I am going to need to quit power walking for exercise due to plantar fascia so I am turning my attention to bicycling. I live in the mid-west on relative flat terrain and have been using my daughter's bike. ( a 6 speed - I am using only 1 gear)
Now that I am looking to buy I'm wondering why comfort bikes don't have a rear suspension similar to a mountain bike. It would seem this would absorb rear shock better than springs in the seat. I'm thinking of maybe customizing a mountain bike to make it like a comfort bike. But maybe a suspension front fork and a cushy spring seat will be enough?
Any suggestions would be greatly appreciated.

Answer
The suspension on mountain bikes is meant to absorb the bumps and rough terrain that these bikes are designed for. Street riding is much smoother and doesn't need the extra suspension. Unless you are planning on taking the bike off road a "comfort" bike would be a better choice. It will give you an easier ride with thinner tires and you don't need the suspension for street riding.

Hope this helps
Scott<>