How far can you walk in 20 minutes--a mile?

Typically walkability is defined as less than 600 foot distance to motorized transport.(OTH, bicycles do extend that considerably, though they compete with cars on the street.)
When you say 'walkability' you really mean walk PLUS mass transit, so this ends up as suburban and mass transit are mutually exclusive, a fine bit of circular reasoning.

A lot of 'walkability' issues obviously are related to zoning and there are areas in cities zoned for residences which keep retail or business use a good distance from a residence. This happened because people wanted to keep their homes away from the 'streetscape'.
By your definition, would those areas suddenly become suburban? Most zoning codes neatly segregate residences from commercial or industrial property and getting variances is very difficult.

Mass transit serves a small minority of the population(far under 50%) in every US city except overpopulated New York City and every seat is subsidized.

Many people dislike the noise, smell and dirt of cities and decided to move to suburbs despite miserable commutes to miserable jobs.

Given the Internet, flex-time and a less secure job environment,things like telecommuting, satelite offices, part-time or a three day work week more do-able and save some energy for commuting(which amounts to just 15%(!!) of total miles per households--45% goes for shopping, 27% for visiting friends/recreation)).