Not counting the Leighton Road segment (which isn't necessary - just did this for a quick back to the parking lot). There is a total of 3.2 mi of road on that map. Here are the road segments:
"official" road connection on East Branch Trail (Falmouth Rd) is 0.2 mi (actually this isn't on the map)
alternative 1 (Terra Way): 0.54 mi
alternative 2 (Mcrann field): <0.05 mi
road connection between East Branch Trail and Community Park via Huston Park (Falmouth Rd): 0.6 mi
road connection between East Branch Trail and Community Park via Twin Pond Road: 0.7 mi
road connection between Corsetti's trailhead of North Side Presumpscot R. trail and Riverton Trolly Park trails: 0.3 mi
road section of River Trail along Riverside Rd (much of this is grass shoulder and dirt road): 0.68 mi
Blackstrap Rd trailhead of South Side Presumpscot River Trail and RR: 0.22 mi
Some other connections not on map:
road connection between Falmouth High School trailhead of East Branch Trail and Field Road trailhead of community park trail (not on map): 1.03 mi
Gray Road-Trolly Farm Way connector between Hadlock and Blackstrap (Ian's suggestion): 1.2 mi
Corsetti's trailhead to Duckpond trailhead (Hardy Rd. system): 1 mi
Ian's loop would have about 1 + 1.2 + 0.7 = 2.6 miles of road. Another 10-15 miles of trail could be added with no additional road. To avoid the poison ivy would require an additional 1 road mile (using the south side river trails).
Another adventure long run would be point-to-point starting at Lowell Preserve in Windham and ending at Back Cove.