Going to increase my two-hour run

During the winter I ran a weekly 2 1/2 hour long run (12-13 miles) and a 2 hour medium run (9-10 miles). About 3 weeks ago, I converted the long run to a one-hour speed run, initially focusing on fartleks. I've been concerned that I might be losing my ability to run more than 10 miles, and I've decided to slowly add half an hour to the medium run. I'll add 5 minutes per week, and in 6 weeks that run will be up to 2 1/2 hours (12-13 miles). In doing this, I'll be adding stress to my body from two sources (speed and distance). I usually only add stress from one source at a time, but I'm hoping that the stress from the small 5 minute increment won't overload my body. We'll see.