Working on Rebuilding Running Endurance

I've been increasing my outdoor running workouts as I gradually expand my own created course around my neighborhood and the adjacent ones here in far northern Gainesville.  Yesterday I covered 5.5 miles in midday 93-degree conditions while taking regular interspersed walking breaks along with a sports drink to sip on.  Although it got a little grueling at the end with the heat, I feel I made some good progress with my running and plan to go out again likewise tomorrow morning...maybe for a little bit longer as I continue to build up my distance.  I used to have a course that stretched through Northwood Pines, Northwood Oaks, the NW 53rd Avenue trail between 34th and 43rd Streets, NW 37th Street and the neighborhoods past that church, NW 43rd Street past Talbot Elementary and into the subdivision on 62nd Avenue, Felasco Park past the TV-20 WCJB station, NW 52nd Terrace, NW 53rd Avenue to the shaded, winding neighborhood of big homes off NW 57th Way...and back again.  Depending on how long I felt like running (or had some outside limit placed on me such as actually having to go to work), I could end up running quite some distance, topping 15 miles a number of times and achieving 26.6 once (at 4 hours 23 minutes).  But my long distance running endurance has admittedly dwindled a bit over the past three or four years and I'd like to return it to where I was running down all those convoluted roads and pathways and racking up the big miles: should be interesting...