Intervals will help restore your lean body mass to some degree, and certainly more so than the muscle-wasting endurance running.
Have you considered doing some strength training, too (say 2-3 sessions x 30 min / week or so)?
Strength training and good nutrition = more testosterone = muscle = more lean, which will also benefit your health in other ways.
As a former runner (up to 35 miles/week, inc. half-marathons), I can atest to both sides of the story. High intensity strength training and "cardio" are much better for your lean body mass.