No, I do not think there is an easy button to navigate to the nearest portal.
In my experience, the scanner view will load portals within a range of approximately 450 yards 0.25 miles, or 0.4 km.
If you are moving, sometimes the scanner will show a rectangular portal marker to a portal that was recently inside scanner range. The "Navigate to portal" button on the portal view will lock the portal marker on your scanner screen, but you can only have one active nav portal at a time.
Portal keys in your inventory provide a remote portal view, and you can zoom out a little to see the scanner range around the remote portal.
The official intel map is the best tool for finding the nearest portal when nothing is within scanner range. I stick with mobile version on my phone, and always click "go to my location" (compass crosshairs in the top right corner). Save a bookmark to the intel map on your home screen, so it is just as easy to pull up as your scanner.
I also check the intel map on my desktop before heading out to mentally plot a course. The full site has more functionality, but still loads frustratingly slow.