Running on grass for track training.

Personally I think training on grass is superior to the track (for distance) but loads of people disagree with me.

For easy mileage grass or dirt trails are great. Especially if you live near trails with good elevation gain, you'll get a good strength training stimulus in too. And don't worry about the 20 second difference, just focus on running relaxed and easy on easy days.

nothing at all wrong with running most if not all mileage on grass. Some would say that every once in a while you need to run on a firm, fixed surface in order to be able to race later on the roads or on a track but if you can avoid hard surfaces good for you. In HS I would run once/week around a half mile field for an hour to avoid pounding roads with mid 1970's shoes and once/week on the beach without shoes. Barefoot running on beach or grass builds up your feet and lower legs in a way you can't achieve when your feet are in shoes

Yes it matters. Recruitment patterns change with velocity.

If soft surface keeps you uninjured though, there's a pretty good trade off there. You don't really need pavement for training ever.

If you have a course w some rougher sections, just set up your speed variations for the best/ smoothest stretches. Walking and jogging where speed is least attainable/ then open it up again where you can safely.