Once the runs start moving above 80% it gets hard to do them on grass. There are very few fields that I know of that can accommodate a split 600m without making for some tight turning, which in turn leads to the possibility of another series of leg issues.
We have access to a soccer pitch Mon-Thu, but it’s condition drops dramatically as the season progresses. Saturdays and some Fridays we can get on the baseball field which is huge (400m around). The grass tends to be better kept there.