Re-directing to front-end after login
As far as I understand it, the postLoginRedirect only affects login forms defined on the the front-end. If a user visits the login page (through a 'login' link for example), then they will be redirected to 'postLoginRedirect' after logging in. They will also be redirected there if they attempt to visit the login form directly when already logged in.

Your best option is to just create a quick login page template, like the one provided in the docs. Craft will automatically look for 'login.html' in your templates folder - otherwise you can specify the location using the loginPath config setting.

With that defined, if a user tries to visit any page that has {{ loginRequired }} specified, they will automatically get redirected to the login page and subsequently be redirected back to the original page request after login.