Can't Swim... Need Training Help

I would say yes definitely do something on the "swim days". But feel your legs as well so you don't overdo it, any sign of pain just reduce the amount again. They need some time to recover between each hard training.
Maybe an idea could be to add one day of brick training (bike first, then immediately run) - the bike part could be half of distance/time of the longest bike session you currently have per week, and then just do a 15-20 minute light run after.
On the other day you could do some core or upper body training so you get some rest for your legs.