Yes, there is a pathway between the Poly and GF. Its been there for quite a long time. Depending on how fast you walk, I'd expect it to take 15+ minutes.
We love to take the walk in the mornings, very tranquil, about 10 to 15 minutes as others have said(always admire the Wedding Chapel). Sometimes we will do the walk late morning and have lunch or breakfast at Gaspirillos(Quick Serve) and walk the grounds. I love salads and I love the made to order salads there, delicious and a good size portion!!
Here's a Saturday Stroll video that one of the DIS Unplugged podcasters made that will show you the scenery on the path. He filmed going from the GF to the Poly.
I had to look it up. From the Disney website:
"Approximately 1 mile long each way, it connects the 2 Resorts, passes Disney’s Wedding Pavilion and offers a view of Magic Kingdom park."