Hi ST,
I think that looks alright. I did a couple of 18-19 for my mara last year. As Maths says, you don't want to get injured. Is the last 10 miler the week before the mara. The 20 will be hard but it is the killer test. Use it for testing clothing and nutrition so on the day you're all set.
If you're desperate to do more, throw in some easy bike rides as a recovery. Make sure they're S L O W. You could also another easy swim in the week if you need more recovery.
Don't forget to eat the right stuff and sleep well.