[mom-from-missouri]

Check out the local community centers in towns--for example, in my town, the community center has an indoor pool and exercise room which you do pay for, but anyone can use the walking track and restrooms for free--and the restrooms all have showers. If you want a locker, though, its a quarter. In the city, they have YMCA's with the same setup.
If you are driving during the summer, you may want to stop and swim every other day for a break and to relax those muscles from driving. Pools always have showers too.