I've logged 156 workouts since Jan. 1, with about 10 days of total rest, so 35 days have been double workout days. Most of those double dip days are commutes where each 20 mile stretch counts as a workout. No issues yet (actually started this intensity in September last year, trying to accelerate rehab from a broken hip). Running 70 to 100 miles a week 35 years ago, on the other hand, left me constantly injured.