We did it! This was also a celebration of us having moved to London exactly 2 years ago, to the day.
We took the GWR from Paddington to Windsor and then started running (after the obligatory photo of the old English Castle). For anyone planning on doing this, FYI, the introduction of cars and asphalt to the world has made the section from mile ~4 to ~6 on Uxbridge road harder in the sense that it's a highway and there's no sidewalk. Yep. Otherwise it was good and easy to navigate, not necessarily the most beautiful way to get between these two places, but the historical element is very cool and you do see quite a few interesting neighborhoods. It ends near a Tesco local and a few underground stations so it was easy to grab some food after finishing and then get back home.
We did it completely by ourselves, supported only by the food and drink in our running backpacks.