Need an ability to restrict proximity search distance in exposed view filter.

Hello,

I've been playing around with the Distance/Proximity search feature for views and haven't been able to figure this one out. I'm in need of restricting results to always be no more and no less than 20 miles, but user must be able to specify a zip code.

So what would be available to user is a field to input a zip code, and that's it. System would then display results based on a default/predetermined distance where user cannot change that distance setting either via page or URL.

This *might* not be a huge issue if I could simply hide the distance input field via CSS, but I can change that value in the URL. At any rate it would be best if the user wouldn't have the ability to set the distance value at all in any way.

Any thoughts?