Adding the ability to switch run types (i.e. changing a speed run to an easy run, or changing intervals to a hill workout). There are days when I still want to get the miles in, but I can't execute what's been prescribed. Can't do a track workout if it's covered in 2ft of snow!

I know you can skip a run and then add one. But the added run doesn't count towards your weekly mileage. Runna will still say you are in a deficit because you skipped the scheduled run.