On foot in Tuscany countryside towns

That is not always the case--often we parked just steps outside the ztl.
If walking is a concern, I would zoom in on google maps to view parking options and seek out bus info.
Taxis often require prebooking.

It varies from town to town. Most of the towns are fairly small and easy to explore. Here's a list of towns we visited.

Greve - there's no ZTL since it is not a hill town. There's a big parking lot just a minute or so from the town square.
Radda - this lovely hill town has parking all around the town's wall, which defines the ZTL. We parked 25 feet from the main gate.
San Gimignano - there's a big parking lot to the left of the main gate, which is maybe 100 feet away.
Volterra - there is underground parking just outside the main gate, with steps that lead directly inside the walls. There's also parking outside the ZTL on the other side of town.

Montepulciano - We didnt' go there, but there is nearby parking outside the ZTL. This may be the most challenging to get from parking to inside the walls.

When will you be there? We spent 10 days in Tuscany December 2015. Since it was off season, we had no problems finding a place to park. If you come at the height of the season, unless you get there early, parking may be harder to come by.

For towns covered in Rick's Italy guidebook, he'll generally recommend parking locations and tips for avoiding steep hills. Many towns have some combination of shuttle bus, escalator, or elevator to get up at least part of the hill. Montepulciano has a bus, for instance. In Montalcino, parking lots near the Fortezza are on the "top ridge level," while some others are further down the side of the hill, with a staircase and possibly no escalator option. Each town's tourist information office may have relevant maps online, too.