Staying motivated during longer runs

I've been running pretty much my whole life but started off as a sprinter until my mid twenties before switching to longer distances. Now I (33F) am hoping to complete a half marathon within a year. I've got up to consistently running about 7 miles 2 to 3 times a week, and over six days average 25 to 30 miles but when I try to increase mileage over more days it's as though my head turns to mush maybe 0.5 to 1 mile into the run and I resort to sprint intervals as an alternative. For example, I ran 7 miles yesterday, aimed for a 10km today but just couldn't focus on keeping going. Instead I did around 3 miles of walk/sprint intervals. I don't think it's a case of fitness, my body was happy to run but just needed to go fast then slow rather than a consistent pace, if that makes sense. Is there a trick to building up to consistent longer runs across more days? Thanks!