Go into Google maps
http://maps.google.co.uk/maps?hl=en&tab=wl
Display the map of the place you want to walk (Type Euston London or similar).
On the left select "Get Directions" and then select Walking in place of By Car.
Find the place where you want to start your walk, RIGHT click and select "Directions from Here"
Now find the place where you want to END you walk, and RIGHT click and select "Direction to Here".
Google will "draw" the route on the map, and give you distances and times for doing the walk.
Says it is about 1.5 miles