Haven't been in there in about 6 years, but the Trumbull run (Railroad Bed) listed above can easily be stretched to 10-12 miles. Follow the bed from the Churchill Rd. entrance listed above. After a mile and a half it will cross whitney Ave, stay on the bed/trail until you go under Rt. 25 and into the Old tungsten Mine Park. Head across the bridge on your right and up the hill. Keep to the left and it will circle the mines up in the hills and bring you back to the bridge. Head back along the bed/trail to Whitney Ave. Take a left on Whitney and head up the hill. there will be an acess to the blue trail on your right. If you miss it, continue on to Indian Ledge Park. There is another acess to the trail on your right just behind the sprinklers/kiddie park. These trails lead you back to trumbull Center on the opposite side of the river. Follow until you hit Daniels Farm Rd. Take a right to head back to your car. I did a lot of running in there in the 60's-90's.
You can also try the reservoir property. Follow Madison ave to Seeley Rd. in Trumbull. At teh end of Seeley is a beautiful trail that will bring you all the way out to Tashua Rd./Velvet St. Don't get caught!! I used to ell the res people I was doing deer surveys for school.